72. Pregnancy and Parturition

Introduction

• Pregnancy begins with implantation of the developing embryo and is sustained by complex maternal, placental, and fetal adaptations. This chapter explores the physiological changes of pregnancy, the role of placental hormones, and the neurohumoral mechanisms that initiate parturition.

• Pregnancy begins when a sperm fertilizes a mature oocyte, followed by implantation of the developing embryo in the uterus.
• Successful conception depends on the timing of intercourse relative to ovulation.
• Spermatozoa can remain viable within the female reproductive tract for approximately 3–5 days under favorable conditions.
• The fertilizing capacity of the ovulated oocyte is usually limited to about 12–24 hours after ovulation. Therefore, the highest probability of conception occurs when intercourse takes place during the fertile window, which includes the days preceding ovulation and the day of ovulation.
• In a regular 28-day menstrual cycle, ovulation typically occurs around the 14th day, although considerable individual variation exists.
• Pregnancy is confirmed only after successful implantation of the embryo in the endometrium.

Physiology Of Pregnancy

Transport of Gametes

Transport of Egg

• Following ovulation, the secondary oocyte is released onto the ovarian surface or into the peritoneal cavity near the ovary.
• The fimbriae at the distal end of the uterine tube actively capture the oocyte and guide it into the tubal lumen.
• The fimbriae contain smooth muscle fibers and are lined by ciliated epithelium.
• Shortly after ovulation, smooth muscle contractions bring the fimbriae closer to the released oocyte.
• Coordinated beating of the cilia creates currents that direct the oocyte toward the interior of the uterine tube.
• These ciliary movements help sweep the oocyte into the tube soon after its release from the ovarian follicle.
• Within the uterine tube, transport of the oocyte occurs through the combined action of ciliary activity and gentle muscular contractions of the tubal wall.
• Movement of the oocyte is relatively slow, and it usually reaches the uterine cavity in about 3–4 days.
• Because the unfertilized oocyte remains viable for only about 12–24 hours after ovulation, fertilization normally occurs in the ampulla, the distal part of the uterine tube.
• Successful fertilization therefore requires timely transport of both gametes within the female reproductive tract.

Transport of Sperm

• During sexual intercourse, semen is deposited in the vagina near the external opening of the cervix.
• Following ejaculation, spermatozoa begin their journey through the female reproductive tract toward the site of fertilization.
• Although the usual transit time to reach the uterine tubes is approximately 4–6 hours, some spermatozoa can be detected in the uterus and uterine tubes within minutes.
• A large proportion of spermatozoa do not survive because the vaginal environment is relatively acidic, with a pH of about 3.8–4.5 under normal conditions.
• Seminal fluid temporarily buffers vaginal acidity, thereby improving sperm survival.

Transport of Sperm in the Cervix

• The cervix acts as a selective gateway that facilitates the passage of healthy and motile spermatozoa.
• Transport through the cervix is enhanced by estrogen-induced changes in cervical mucus during the periovulatory period.
• Under the influence of estrogen, cervical mucus becomes thin, watery, and highly elastic.
• The mucus also develops parallel channels that guide spermatozoa toward the uterine cavity.
• These changes reduce resistance to sperm movement and improve the likelihood of successful fertilization.

Transport of Sperm in the Uterus

• Sperm transport through the uterus occurs by a combination of sperm motility and rhythmic uterine contractions.
• Uterine contractions help propel spermatozoa toward the uterine tubes.
• These contractions are influenced by female reproductive hormones and substances present in semen.

Transport of Sperm in the Fallopian Tube

• Within the uterine tube, sperm movement is assisted by ciliary activity of the tubal epithelium.
• Peristaltic contractions of the tubal wall also contribute to sperm transport.
• The flow of tubal fluid further facilitates sperm movement toward the ampulla, the usual site of fertilization.
• Although hundreds of millions of spermatozoa may be ejaculated, only a few hundred reach the ampullary region. Therefore, adequate sperm count, normal motility, and proper sperm function are essential for successful fertilization.
• Reduced sperm concentration or impaired motility can contribute to male infertility.

Changes in Sperm in Female Genital Tract

• Spermatozoa undergo functional changes within the female reproductive tract that enable them to fertilize the secondary oocyte. These changes are collectively known as capacitation.
• Capacitation usually occurs over several hours after spermatozoa enter the female genital tract.
• During this process, the pattern of tail movement changes from gentle, wave-like beating to vigorous whip-like movements.
• This enhanced motility, known as hyperactivation, improves the ability of spermatozoa to move through the reproductive tract and penetrate the egg coverings.
• Capacitation also produces important alterations in the sperm plasma membrane.
• These membrane modifications prepare the spermatozoon for binding to and fusing with the oocyte.
• Freshly ejaculated spermatozoa cannot normally fertilize an oocyte immediately.
• Only capacitated spermatozoa acquire the ability to penetrate the egg and achieve fertilization.

Fertilization

• Fertilization is the process by which a spermatozoon and a secondary oocyte unite to form a diploid zygote. It normally occurs in the ampulla of the uterine tube.
• Fertilization involves a sequence of events that includes sperm binding, acrosome reaction, sperm penetration, prevention of polyspermy, and zygote formation.

Fusion of Sperm with Egg

• The secondary oocyte is surrounded by the corona radiata and the zona pellucida.
• Numerous spermatozoa pass through the corona radiata and reach the zona pellucida.
• The zona pellucida contains specific glycoproteins, particularly zona pellucida glycoprotein 3, which act as receptors for sperm binding.
• Receptor-mediated binding ensures species-specific recognition between spermatozoa and the oocyte.
• Only capacitated spermatozoa can effectively bind to the zona pellucida and participate in fertilization.

Acrosome Reaction

• Binding of a capacitated spermatozoon to the zona pellucida triggers the acrosome reaction.
• During this process, the plasma membrane overlying the sperm head fuses with the outer acrosomal membrane. This fusion increases membrane permeability and allows the release of acrosomal enzymes.
• Important enzymes include acrosin and other hydrolytic enzymes that digest a pathway through the zona pellucida.
• Simultaneously, calcium influx into the spermatozoon enhances its motility and facilitates penetration.
• The spermatozoon advances through the zona pellucida by the combined action of enzymatic digestion and vigorous tail movements.
• After crossing the zona pellucida, the spermatozoon enters the perivitelline space, which lies between the zona pellucida and the oocyte plasma membrane.
• The sperm cell then reaches the oocyte membrane and establishes contact with it.
• Fusion of the sperm and oocyte membranes is mediated by complementary membrane proteins present on both cells.
• Following membrane fusion, the sperm head and tail enter the oocyte cytoplasm.
• Entry of the spermatozoon activates the oocyte and initiates the subsequent events that prevent polyspermy and lead to formation of the zygote.

Polyspermy Block

• Normally, only one spermatozoon fertilizes the secondary oocyte.
• Entry of additional spermatozoa is prevented by mechanisms collectively known as the polyspermy block.
• These mechanisms are activated immediately after fusion of the sperm and oocyte membranes.
• The first mechanism is a rapid change in the electrical properties of the oocyte membrane.
• This transient membrane depolarization reduces the likelihood of additional sperm–oocyte fusion.
• The second and more important mechanism is the cortical reaction.
• Cortical granules located beneath the oocyte plasma membrane release their contents into the perivitelline space by exocytosis.
• Enzymes released from these granules modify the zona pellucida.
• These modifications inactivate sperm-binding sites and alter the structure of the zona pellucida.
• As a result, the zona pellucida becomes impermeable to additional spermatozoa. This process, known as the zona reaction, ensures that only one paternal genome enters the oocyte.
• Prevention of polyspermy is essential for maintaining the normal diploid chromosome number in the developing embryo.

Zygote Formation

• Following sperm entry, the secondary oocyte completes meiosis II.
• Completion of meiosis results in the formation and extrusion of the second polar body.
• The remaining haploid female nucleus develops into the female pronucleus.
• Simultaneously, the sperm nucleus enlarges and forms the male pronucleus.
• Each pronucleus contains 23 chromosomes enclosed within a separate nuclear membrane.
• The male and female pronuclei migrate toward the center of the oocyte with the assistance of cytoskeletal elements, including microtubules.
• During this period, deoxyribonucleic acid replication occurs within both pronuclei.
• The pronuclear membranes subsequently break down.
• Maternal and paternal chromosomes then align on a common mitotic spindle.
• Fusion of the genetic material from both gametes restores the diploid chromosome number of 46. This event is termed syngamy and marks the completion of fertilization. The resulting cell is called the zygote, which contains a unique genetic combination derived from both parents.
• The zygote soon undergoes its first mitotic division and initiates embryonic development.

Cleavage

• Cleavage is the series of rapid mitotic divisions that occurs after fertilization as the zygote travels through the uterine tube toward the uterus. These divisions increase the number of cells without increasing the overall size of the developing conceptus. Consequently, the conceptus remains approximately the same size as the original zygote despite repeated cell divisions.
• The daughter cells produced during cleavage are called blastomeres.
• The first cleavage division usually occurs about 24–30 hours after fertilization and forms a two-cell stage.
• Subsequent divisions produce four-cell and eight-cell stages over the next few days.
• By approximately 3–4 days after fertilization, the conceptus develops into a morula, which consists of about 16–32 blastomeres.
• In some cases, early embryonic cells separate and develop independently, resulting in monozygotic twins.
• During cleavage, the conceptus gradually moves from the ampulla of the uterine tube toward the uterine cavity.
• Until implantation, the developing conceptus remains enclosed within the zona pellucida.
• The zona pellucida prevents premature attachment of the conceptus to the uterine tube and thereby reduces the risk of ectopic implantation. It also protects the conceptus from mechanical injury during transport through the reproductive tract.
• In addition, the zona pellucida provides a protective barrier that helps shield the early conceptus from maternal immune recognition.
• The morula typically enters the uterine cavity about 3–4 days after fertilization, where further development continues.

Blastocyst Formation and Implantation

• The morula enters the uterine cavity about 4 days after fertilization. It remains free within uterine secretions for approximately 2–3 days while further cell divisions occur.
• During this period, a fluid-filled cavity develops, and the conceptus becomes a blastocyst.
• At the blastocyst stage, cellular differentiation begins and individual cells are no longer totipotent.
• The blastocyst derives nourishment from uterine glandular secretions before implantation.
• The blastocyst consists of three major components.
• The trophoblast forms the outer cell layer of the blastocyst. It plays a key role in implantation and later contributes to formation of the fetal portion of the placenta.
• Trophoblastic cells secrete human chorionic gonadotropin, which helps maintain early pregnancy.
• The embryoblast or inner cell mass is located on one side of the blastocyst. It gives rise to the embryo and subsequently the fetus.
• The blastocyst cavity (blastocoel) is the fluid-filled central cavity of the blastocyst. It facilitates blastocyst expansion and early embryonic organization.
• During subsequent development, the embryoblast forms the embryo, whereas the trophoblast contributes to placental development, fetal nutrition, and hormone production throughout pregnancy.

Implantation and Early Development

• Implantation is the process by which the blastocyst attaches to and becomes embedded within the endometrium of the uterus. It usually begins about 6–7 days after fertilization and is established by approximately 10–12 days after fertilization.
• Successful implantation requires a receptive endometrium that has been prepared by progesterone and estrogen.
• At this stage, the menstrual cycle is in the secretory phase, during which the endometrium becomes thick, vascular, and glandular.
• Progesterone secreted by the corpus luteum plays a major role in preparing the uterus for implantation.
• The blastocyst commonly implants in the upper posterior or anterior wall of the uterine body.
• Implantation outside the uterine cavity results in ectopic pregnancy, a potentially serious clinical condition.

Mechanism of Implantation

• Implantation depends on coordinated changes in both the blastocyst and the endometrium.
• Molecular interactions between trophoblastic cells and endometrial cells facilitate attachment and invasion.
• These interactions ensure that implantation occurs only during a limited period known as the implantation window.

Changes in Blastocyst

• Before implantation, the blastocyst enlarges and sheds the zona pellucida through a process called hatching.
• Removal of the zona pellucida allows direct contact between trophoblastic cells and the endometrial surface.
• Trophoblastic cells possess adhesion molecules that promote attachment to the endometrium.
• Initial contact stimulates rapid proliferation and differentiation of trophoblastic cells.

Interaction of Blastocyst with Endometrium

• Trophoblastic cells secrete proteolytic enzymes that digest the extracellular matrix of the endometrium.
• These enzymes enable the blastocyst to penetrate and become embedded within the uterine lining.
• Microvilli of trophoblastic cells establish close contact with endometrial cells, strengthening implantation.

Changes in Endometrium

• Under the influence of progesterone, the endometrium undergoes decidualization.
• Endometrial stromal cells enlarge and accumulate glycogen and lipid reserves.
• The modified endometrium is known as the decidua.
• During the early weeks of development, decidual cells provide nutrients to the embryo.
• As pregnancy progresses, the placenta gradually assumes the major role in fetal nutrition, gas exchange, and hormone production.

Placentation and Amnion Formation

Placenta Formation

• Implantation is usually completed about 10–12 days after fertilization.
• Following implantation, trophoblastic cells differentiate into two layers: the cytotrophoblast and the syncytiotrophoblast.
• The cytotrophoblast consists of distinct cellular layers, whereas the syncytiotrophoblast forms a multinucleated layer without clear cell boundaries.
• Maternal endometrial blood vessels enlarge and form blood-filled spaces known as lacunae.
• Primary chorionic villi begin to develop during the second week of embryonic development.
• These villi subsequently become vascularized and form the functional units of the placenta.
• The placenta is a specialized organ that enables exchange of gases, nutrients, metabolic wastes, and other substances between the mother and fetus.
• The fetal component of the placenta is derived mainly from the chorion, which develops from trophoblastic and extraembryonic mesodermal tissues.
• The maternal component is formed by the decidua basalis of the endometrium.
• Chorionic villi are finger-like projections that extend from the chorion into the maternal decidua.
• Each villus contains fetal capillaries and is bathed by maternal blood within the intervillous spaces.
• Maternal and fetal blood normally remain separated by the placental barrier.

Arrangement of Placental Circulation

• Maternal blood enters the intervillous spaces through branches of the uterine arteries.
• After exchange occurs, maternal blood returns to the maternal circulation through uterine veins.
• Fetal blood reaches the placental villi through two umbilical arteries, which carry deoxygenated blood from the fetus.
• Exchange of oxygen, nutrients, and waste products occurs across the placental membrane.
• Oxygenated blood then returns to the fetus through a single umbilical vein.
• These vessels are enclosed within the umbilical cord, which connects the fetus to the placenta throughout pregnancy.

Formation of Amnion

• As early embryonic development progresses, a fluid-filled space called the amniotic cavity forms above the embryonic disc.
• The cavity gradually enlarges as pregnancy advances. It becomes lined by a thin membrane called the amnion, which is derived from embryonic cells.
• The amnion encloses the developing embryo and later the fetus.
• With further growth, the amnion expands and eventually fuses with the chorion, forming the amniochorionic membrane.
• The cavity contains amniotic fluid, which surrounds the fetus throughout intrauterine life.
• Amniotic fluid protects the fetus from mechanical injury and helps maintain a stable thermal environment. It also permits free fetal movements, which are important for normal musculoskeletal development.
• The fetus remains suspended within the amniotic cavity and is connected to the placenta by the umbilical cord.
• Exchange of gases, nutrients, and waste products occurs across the placental barrier, while maternal and fetal blood normally remain separate.
• By late pregnancy, the amniotic sac forms the principal membrane surrounding the fetus within the uterus.

Techniques for Fetal Sex Diagnosis

• Chorionic villus sampling is a prenatal diagnostic procedure in which a small sample of chorionic villi is obtained from the placenta. It can usually be performed between 10 and 13 weeks of gestation.
• The procedure allows early detection of chromosomal abnormalities and certain inherited genetic disorders. However, it carries a small risk of miscarriage and other procedure-related complications.
• Ultrasonography is a noninvasive imaging technique widely used during pregnancy. It provides information about fetal growth, structural development, placental location, and fetal well-being.
• Determination of fetal sex by prenatal diagnostic techniques is legally prohibited in India except when medically indicated.

Organogenesis and Fetal Nutrition

• Once the placenta is established, the fetus receives oxygen and nutrients from the maternal circulation through placental exchange.
• Adequate maternal nutrition is essential for normal fetal growth and development.
• Organogenesis occurs mainly between the 3rd and 8th weeks of embryonic development, when major organs are formed.
• Nutritional deficiencies during this critical period may result in congenital abnormalities.
• Deficiency of folic acid before conception and during early pregnancy increases the risk of neural tube defects.
• Fetal development can also be adversely affected by environmental and maternal factors.
• Exposure to ionizing radiation, certain infections, harmful chemicals, alcohol, tobacco smoke, and some medications may interfere with normal development.
• Such agents are called teratogens because they can cause congenital malformations or functional defects in the developing fetus.

Fetus as a Graft

• The fetus inherits genetic material from both parents and therefore differs genetically from the mother.
• Despite this genetic difference, the maternal immune system normally does not reject the fetus. This phenomenon is often described as the fetus functioning as a semi-allograft.
• Several mechanisms contribute to maternal–fetal immune tolerance.
• Trophoblastic cells express human leukocyte antigen-G, a nonclassical major histocompatibility complex molecule that reduces maternal immune recognition.
• These cells generally do not express the highly polymorphic major histocompatibility complex class II molecules that strongly stimulate immune responses.
• The placenta also promotes immune tolerance by inducing apoptosis or functional suppression of activated maternal immune cells.
• In addition, the placental barrier limits direct contact between maternal and fetal tissues, thereby reducing immune-mediated injury.
• Together, these mechanisms help maintain a successful pregnancy.

Fetoplacental Unit

• The fetoplacental unit refers to the coordinated hormonal interaction between the fetus and the placenta.
• The placenta synthesizes pregnenolone from maternal cholesterol and produces large amounts of progesterone.
• Some placental pregnenolone enters the fetal circulation and serves as a precursor for steroid synthesis in the fetal adrenal glands.
• The fetal adrenal glands produce dehydroepiandrosterone sulfate and related steroid precursors.
• The fetal liver further modifies some of these precursors through hydroxylation reactions.
• These steroid precursors return to the placenta, where they are converted into estrogens.
• Dehydroepiandrosterone sulfate is converted mainly into estradiol, whereas 16-hydroxylated precursors contribute predominantly to estriol formation.
• This metabolic cooperation between the fetus and placenta is essential for normal steroid hormone production during pregnancy.

Clinical Significance

• Estriol is the principal estrogen produced during pregnancy through cooperation between the placenta and fetal adrenal glands.
• Maternal estriol levels provide an indirect indicator of fetal and placental well-being.

Female Infertility

• Infertility is defined as the inability to achieve pregnancy after 12 months of regular unprotected sexual intercourse.
• Female infertility may result from abnormalities at different levels of the reproductive system.
• Ovarian factors, particularly anovulation, prevent release of a mature oocyte and thereby hinder fertilization.
• Peritoneal factors, such as pelvic adhesions or chronic pelvic inflammation, may interfere with normal capture of the oocyte by the uterine tube.
• Tubal factors, including unilateral or bilateral tubal obstruction due to salpingitis or scarring, impair transport of gametes and fertilization.
• Uterine factors, such as endometrial inflammation, intrauterine adhesions, fibroids, or congenital uterine anomalies, may reduce implantation success.
• Cervical factors include abnormal cervical mucus or antisperm antibodies that impair sperm passage through the cervix.
• Vaginal factors, including infections or structural abnormalities, may hinder sperm transport to the cervix.
• Accurate evaluation of the underlying cause is essential for appropriate management and treatment.

In-Vitro Fertilization and Transfer (IVF-ET)

• In vitro fertilization and embryo transfer is an assisted reproductive technique in which fertilization occurs outside the body under laboratory conditions.
• Modern protocols commonly use controlled ovarian stimulation to obtain multiple oocytes and improve success rates.
• Oocyte retrieval is usually performed under ultrasound guidance through the vaginal route.
• The resulting embryo is transferred into the uterine cavity to achieve pregnancy.

Patient Selection

• Suitable candidates generally have adequate ovarian reserve, a normal uterine cavity, and no active human immunodeficiency virus or hepatitis infection.
• Semen parameters should be evaluated as part of infertility assessment.

Principal steps of an ART cycle

• The cycle includes pituitary suppression or regulation, controlled ovarian stimulation, and monitoring of follicular development.
• Mature oocytes are retrieved, fertilized in vitro, and embryos are transferred to the uterus.

• Progesterone supplementation is provided during the luteal phase to support implantation and early pregnancy.

Oocyte Retrieval

• Oocyte retrieval is performed under aseptic conditions through the vaginal route using transvaginal ultrasonographic guidance.
• The procedure is usually scheduled about 34–36 hours after administration of human chorionic gonadotropin and before ovulation occurs.
• Intravenous sedation and analgesia are commonly used to ensure patient comfort.
• Retrieved oocytes are identified by the surrounding cumulus cell complex.
• After collection, oocytes are maintained in specialized culture media for several hours before fertilization.

Fertilization in Vitro

• Semen samples are processed before insemination using density-gradient centrifugation or swim-up techniques to select motile spermatozoa.
• Prepared spermatozoa are added to culture media containing mature oocytes within a few hours of retrieval.
• Fertilization is usually assessed 16–18 hours later.
• Successful fertilization is indicated by the presence of two pronuclei and a second polar body.
• Adequate sperm concentration and motility are important determinants of successful fertilization.
• Semen is generally collected on the day of oocyte retrieval to ensure optimal sperm quality for the procedure.

Embryo Transfer

• During embryo transfer, one or more embryos, commonly at the cleavage stage, are placed into the uterine cavity through a soft transcervical catheter.
• The procedure is usually performed 3–5 days after fertilization.
• Limiting the number of transferred embryos reduces the risk of multiple pregnancy.

Placental Hormones

• The placenta functions as an important endocrine organ during pregnancy and secretes several hormones essential for fetal growth and maintenance of gestation.
• In early pregnancy, the corpus luteum remains active and produces progesterone and other hormones required for continuation of pregnancy.
• As placental hormone production increases, the endocrine role of the corpus luteum gradually declines.
• By approximately 8–10 weeks of gestation, the placenta becomes the principal source of progesterone.
• Consequently, removal of the ovaries after this period usually does not interrupt an established pregnancy.

hCG

• Human chorionic gonadotropin is a glycoprotein hormone secreted by the syncytiotrophoblast of the developing placenta.

• Production begins about 6–8 days after fertilization, shortly after implantation. It is one of the earliest detectable markers of pregnancy.

Structure

• Human chorionic gonadotropin consists of two glycoprotein subunits, designated α and β.
• The α-subunit is structurally similar to the α-subunits of luteinizing hormone, follicle-stimulating hormone, and thyroid-stimulating hormone.
• The β-subunit is unique and confers the biological and immunological specificity of human chorionic gonadotropin.
• Measurement of the β-subunit forms the basis of most pregnancy tests.

Secretion

• Human chorionic gonadotropin is secreted in large amounts during early pregnancy by the syncytiotrophoblast. It can be detected in maternal blood approximately 6–8 days after fertilization, shortly after implantation.
• Circulating levels rise rapidly during the first trimester.
• Concentrations usually reach a peak around 9–12 weeks of gestation.
• After the first trimester, levels gradually decline and then remain relatively stable for the remainder of pregnancy.
• As human chorionic gonadotropin levels decrease, placental production of progesterone and estrogens increases and becomes sufficient to maintain pregnancy.

Functions

• Human chorionic gonadotropin binds primarily to luteinizing hormone receptors in the corpus luteum. It maintains the corpus luteum during early pregnancy and stimulates continued progesterone secretion. This action prevents menstruation and supports implantation and early embryonic development.
• Human chorionic gonadotropin also contributes to placental steroid hormone synthesis.
• In male fetuses, it stimulates fetal Leydig cells to produce testosterone, which is essential for normal sexual differentiation.
• Elevated levels of human chorionic gonadotropin are associated with nausea and vomiting commonly experienced during early pregnancy.

Clinical Significance

• Measurement of human chorionic gonadotropin forms the basis of most pregnancy tests.
• The hormone can be detected in blood earlier than in urine.
• Urinary detection becomes reliable approximately 10–14 days after conception.
• Serial measurement of human chorionic gonadotropin is useful for assessing early pregnancy progression.
• Abnormally high or low levels may indicate pathological conditions such as trophoblastic disease, ectopic pregnancy, or pregnancy loss.
• Although strongly associated with pregnancy, elevated human chorionic gonadotropin may also occur in certain germ-cell and trophoblastic tumors.

hCS

Source

• Human chorionic somatomammotropin, also known as human placental lactogen, is secreted by the syncytiotrophoblast of the placenta.

Structure and Secretion

• Human chorionic somatomammotropin is structurally related to growth hormone and prolactin. It is a single-chain polypeptide hormone containing 191 amino acids.
• Secretion begins early in pregnancy and increases progressively as the placenta enlarges.
• Maternal blood concentrations continue to rise throughout gestation and reach maximum levels near term.
• The circulating concentration is closely related to placental mass.

Functions

• Human chorionic somatomammotropin promotes development of the mammary glands and prepares the breasts for lactation. Its lactogenic activity is weaker than that of prolactin. It contributes to fetal growth by modifying maternal metabolism and nutrient availability.
• The hormone stimulates lipolysis in maternal adipose tissue, increasing circulating free fatty acids. It reduces maternal glucose utilization and promotes insulin resistance during pregnancy.
• These metabolic adaptations help conserve glucose for transfer to the fetus.
• By increasing fat utilization in the mother and preserving glucose availability, it supports the energy requirements of the growing fetus.
• Human chorionic somatomammotropin therefore plays an important role in coordinating maternal and fetal metabolism.

Clinical Significance

• Maternal serum levels of human chorionic somatomammotropin reflect placental size and functional capacity.
• Reduced concentrations in late pregnancy may indicate placental insufficiency or impaired placental development.
• Measurement of this hormone can provide information about placental health and fetal well-being.

Other Hormones

• Relaxin is produced by the placenta and corpus luteum during pregnancy. It promotes uterine relaxation in early pregnancy, supports implantation, and helps maintain gestation.
• Near term, relaxin softens the pubic symphysis and pelvic ligaments, facilitating childbirth.
• Progesterone maintains myometrial quiescence, supports endometrial function, and is essential for continuation of pregnancy.
• Estrogens increase progressively throughout pregnancy and reach peak levels near term.
• Estriol is the predominant estrogen of pregnancy and is produced mainly by the fetoplacental unit.
• Estrogens promote uterine growth, increase myometrial responsiveness, and contribute to the initiation of parturition.

Maternal Changes During Pregnancy

• Pregnancy is associated with extensive physiological adaptations in almost all maternal organ systems.
• These changes support fetal growth and development by ensuring adequate delivery of oxygen and nutrients and efficient removal of metabolic waste products.
• Major adaptations include increases in blood volume, cardiac output, ventilation, renal function, and body weight.

Changes in Blood Volume

• Maternal blood volume increases substantially during pregnancy.
• Total blood volume typically rises by approximately 40–50% above prepregnancy levels. This increase results from expansion of both plasma volume and red blood cell mass.
• The increase in plasma volume begins early in pregnancy and becomes evident during the first trimester.
• Plasma volume rises progressively and reaches a maximum during the second and third trimesters.
• Expansion of plasma volume is mainly due to sodium and water retention mediated by hormonal changes.
• This increase helps maintain uteroplacental perfusion and provides a reserve against blood loss during delivery.
• The extracellular fluid volume also increases significantly during pregnancy.
• Retention of water and electrolytes contributes to this expansion and supports the growing fetus, placenta, and maternal tissues.
• Red blood cell mass increases by approximately 20–30% during pregnancy.
• Enhanced erythropoietin production stimulates erythropoiesis and increases oxygen-carrying capacity.
• The rise in red blood cell mass occurs more gradually than the increase in plasma volume.
• Because plasma volume expands disproportionately, hemoglobin concentration and hematocrit decrease slightly despite increased red blood cell production. This dilutional effect is known as physiological anemia of pregnancy.
• Physiological anemia improves blood flow by reducing blood viscosity and does not indicate true iron deficiency. However, adequate iron intake remains essential to support the increased demand for erythropoiesis during pregnancy.

Hematological Changes

• Hematological adaptations during pregnancy enhance oxygen delivery to the fetus and prepare the mother for blood loss during childbirth.
• Red blood cell mass increases by approximately 20–30% due to enhanced erythropoiesis.
• Because plasma volume increases proportionately more, physiological anemia of pregnancy develops.
• Mild to moderate leukocytosis is common and occurs mainly because of an increase in neutrophils.
• A slight increase in immature neutrophil forms may also be observed.
• Platelet count generally remains within the normal range, although a mild reduction may occur in some healthy pregnancies.
• Pregnancy is associated with a hypercoagulable state, particularly during the third trimester.
• Plasma fibrinogen concentration increases markedly, often reaching about twice the nonpregnant level.
• Levels of clotting factors VII, VIII, IX, and X also rise.
• These changes reduce the risk of excessive bleeding during delivery but increase the tendency for thrombosis.
• In certain pregnancy-related complications, excessive activation of coagulation may lead to disseminated intravascular coagulation.

Changes in Cardiovascular System

Cardiac Output

• The most important cardiovascular adaptation during pregnancy is an increase in cardiac output.
• Cardiac output begins to rise early in pregnancy and becomes markedly elevated by the end of the first trimester. It increases by approximately 30–50% above nonpregnant values, reaching its peak during mid-pregnancy.
• Elevated cardiac output is generally maintained until term. This increase ensures adequate blood flow to the uterus, placenta, and other maternal tissues.

Stroke Volume

• Stroke volume increases by about 25–30% during pregnancy.
• The rise is greatest around mid-pregnancy.
• Increased blood volume enhances venous return, leading to a higher end-diastolic volume and greater stroke volume through the Frank–Starling mechanism.

Heart Rate

• Maternal heart rate gradually increases throughout pregnancy. It typically rises by about 10–20 beats per minute above the prepregnancy rate.
• The increased heart rate, together with the elevated stroke volume, contributes significantly to the rise in cardiac output.

Systolic BP

• Systolic blood pressure usually remains unchanged or increases slightly during pregnancy because of the rise in cardiac output.

Diastolic BP

• Diastolic blood pressure decreases during early and mid-pregnancy, reaching its lowest level around 20–24 weeks.
• The fall in diastolic pressure is primarily due to reduced systemic vascular resistance caused by vasodilatory hormones.
• Diastolic pressure gradually returns toward prepregnancy values near term.
• As a result, pulse pressure may become slightly widened during pregnancy.

LVET and PEP

• The pre-ejection period and left ventricular ejection time are generally shortened because of enhanced cardiac performance.

Regional Blood Flow

• Blood flow increases markedly to the uterus, kidneys, breasts, and skin, supporting fetal growth, maternal metabolism, lactation, and heat dissipation.

Changes in Respiratory System

• Respiratory adaptations during pregnancy enhance oxygen delivery to the fetus and facilitate removal of fetal carbon dioxide.
• The most significant change is an increase in minute ventilation.
• Ventilation begins to rise early in pregnancy and increases by approximately 40–50% above prepregnancy levels by late gestation.
• This increase is primarily mediated by progesterone, which stimulates the respiratory center and increases sensitivity to carbon dioxide.
• Respiratory rate changes little or may increase only slightly.
• The rise in minute ventilation occurs mainly because of an increase in tidal volume.
• Residual volume decreases by about 15–20%.
• Expiratory reserve volume also decreases, resulting in a reduction in functional residual capacity.
• Inspiratory capacity increases because tidal volume rises.
• The diffusing capacity of the lungs remains essentially unchanged in normal pregnancy.
• Although the enlarging uterus elevates the diaphragm during late pregnancy, ventilation is generally maintained effectively through compensatory changes in thoracic dimensions.
• These adaptations produce a mild respiratory alkalosis that facilitates transfer of carbon dioxide from the fetus to the mother.

Changes in Kidney Functions

• Renal adaptations help maintain fluid, electrolyte, and metabolic homeostasis during pregnancy.
• Renal blood flow increases by approximately 35–50%, largely due to systemic and renal vasodilation.
• Increased renal perfusion contributes to a substantial rise in glomerular filtration rate.
• Glomerular filtration rate increases early in pregnancy and reaches about 40–50% above prepregnancy values by mid-gestation.
• The elevated filtration rate is maintained throughout most of pregnancy.
• Increased filtration of glucose may exceed tubular reabsorptive capacity, causing mild physiological glycosuria in some pregnant women.
• Increased urinary excretion of amino acids may also occur.
• Despite the higher filtered load of sodium, the kidneys continue to regulate sodium and water balance effectively.
• Hormonal influences promote retention of sodium and water, leading to expansion of extracellular fluid volume.
• These renal changes support the increased circulatory demands of pregnancy and ensure adequate uteroplacental perfusion.

Changes in GI System

• Gastrointestinal motility decreases during pregnancy, mainly due to the relaxing effects of progesterone on smooth muscle.
• Gastric emptying and intestinal transit become slower.
• Prolonged intestinal transit increases water absorption from the colon, which contributes to constipation.
• As pregnancy advances, the enlarging uterus displaces abdominal organs and increases intra-abdominal pressure.
• Compression of the stomach and relaxation of the lower esophageal sphincter promote gastroesophageal reflux.
• Consequently, heartburn and reflux esophagitis are common symptoms, particularly during late pregnancy.

Changes in Hepatobiliary System

• Hepatic blood flow remains relatively unchanged despite the marked increase in cardiac output.
• Total serum protein concentration decreases because plasma volume expands more than hepatic protein synthesis. This results in dilutional hypoalbuminemia and a reduction in serum albumin concentration.
• Plasma fibrinogen concentration increases as part of the pregnancy-associated hypercoagulable state.
• Serum aminotransferase levels generally remain within the normal range.
• Alkaline phosphatase levels increase, largely because of production by the placenta.
• Maternal plasma lipid and cholesterol concentrations rise significantly during pregnancy.
• These changes help meet the metabolic demands of the growing fetus.
• Progesterone reduces gallbladder contractility and promotes biliary stasis.
• Gallbladder emptying becomes less efficient, leading to increased residual bile volume.
• Estrogen increases cholesterol secretion into bile and alters bile composition.
• Reduced bile acid-mediated cholesterol solubility, together with biliary stasis, increases the risk of cholesterol gallstone formation during pregnancy.

Changes in Endocrine System

Pituitary Secretions

• The anterior pituitary gland enlarges during pregnancy, mainly because of hyperplasia and hypertrophy of prolactin-secreting cells.
• Prolactin levels increase progressively and may reach up to ten times the prepregnancy concentration near term.
• Elevated prolactin prepares the mammary glands for lactation.
• High concentrations of estrogen and progesterone suppress the hypothalamic–pituitary–ovarian axis.
• As a result, secretion of luteinizing hormone and follicle-stimulating hormone decreases, preventing ovulation during pregnancy.
• Growth hormone secretion from the maternal pituitary declines, while placental growth hormone contributes increasingly to maternal metabolism.
• Thyroid-stimulating hormone levels usually remain within the normal range.
• Adrenocorticotropic hormone levels rise during late pregnancy because both the maternal pituitary and placenta contribute to its production.

Thyroid Secretion

• Increased glomerular filtration enhances renal iodide clearance during pregnancy.
• If dietary iodine intake is inadequate, mild thyroid enlargement may occur.
• Estrogen stimulates synthesis of thyroxine-binding globulin, increasing circulating total thyroxine and triiodothyronine concentrations.
• Despite higher total hormone levels, free thyroid hormone concentrations generally remain within the normal range, maintaining a euthyroid state.

Adrenocortical Secretion

• Secretion of glucocorticoids and mineralocorticoids increases during pregnancy.
• Plasma cortisol levels rise because of increased cortisol-binding globulin production and enhanced adrenal activity.
• Aldosterone secretion also increases, promoting sodium and water retention.
• These endocrine adaptations help support maternal cardiovascular function, fluid balance, and fetal growth without producing clinical hypercortisolism in normal pregnancy.

Calcium Metabolism

• Calcium requirements increase during pregnancy to support fetal skeletal development.
• Intestinal calcium absorption rises due to increased production of active vitamin D metabolites.
• Parathyroid hormone and related regulatory mechanisms help maintain calcium balance.
• Total serum calcium may decrease because of hemodilution, whereas ionized calcium usually remains normal.
• Adequate calcium intake is important, particularly during late pregnancy when fetal bone mineralization is most rapid.

Body Weight Gain

• Progressive increase in body weight is a normal physiological feature of pregnancy.
• During the first and second trimesters, weight gain occurs partly because of increased maternal fat storage and expansion of body tissues.
• During the third trimester, rapid fetal growth, enlargement of the uterus and placenta, and increased amniotic fluid contribute substantially.
• Expansion of blood volume and extracellular fluid also contributes to weight gain.
• Excessive or rapid weight gain may occur in preeclampsia because of abnormal fluid retention.

Parturition

• Pregnancy may end in abortion or parturition.
• In India, termination of pregnancy before completion of 28 weeks of gestation has traditionally been termed abortion, whereas delivery after this period is considered parturition.
• A normal human pregnancy lasts about 40 weeks from the first day of the last menstrual period or about 38 weeks from fertilization.
• Delivery occurring at term generally takes place between 37 and 42 completed weeks of gestation.
• Birth before 37 completed weeks is classified as preterm delivery.
• Throughout most of pregnancy, the myometrium remains relatively quiescent under the influence of progesterone.
• Progesterone reduces uterine excitability and helps maintain pregnancy.
• During the final weeks of gestation, the uterus gradually becomes more responsive to contractile stimuli.
• Rising estrogen activity promotes synthesis of connexin proteins within myometrial cells.
• Connexins form gap junctions that enhance electrical coupling between adjacent uterine muscle cells.
• Increased intercellular communication allows coordinated and effective uterine contractions during labor.
• The cervix undergoes progressive remodeling before labor.
• Enzymatic breakdown and reorganization of collagen fibers cause cervical softening, a process known as cervical ripening.
• Increased water content and connective tissue changes further facilitate cervical dilation.
• Relaxin, produced by the placenta and other pregnancy-related tissues, contributes to cervical softening and increased pelvic ligament flexibility.
• Estrogen also increases the number of oxytocin receptors in the myometrium.
• Enhanced oxytocin responsiveness strengthens uterine contractions and assists the onset and progression of labor.
• These coordinated hormonal and structural changes prepare the uterus and birth canal for successful vaginal delivery.

Stages of Labor

• The onset of labor is indicated by regular, painful uterine contractions that progressively increase in frequency and intensity.
• A blood-tinged mucous discharge, known as the show, commonly appears due to cervical changes and separation of mucus from the cervical canal.
• Progressive cervical dilation and effacement occur as labor advances.
• Bulging of the amniotic membranes may be observed before rupture of the membranes.

Stage 1

• The first stage extends from the onset of true labor until complete cervical dilatation.
• Cervical effacement and dilatation occur under the influence of coordinated uterine contractions.
• The amniotic sac may protrude through the cervical opening, forming the forebag of membranes.
• Rupture of the membranes usually occurs during this stage or early in the second stage.
• Complete cervical dilatation reaches approximately 10 centimeters.

Stage 2

• The second stage begins after complete cervical dilatation.
• Strong uterine contractions and voluntary maternal bearing-down efforts facilitate fetal descent.
• The fetus passes through the birth canal and is delivered through the vagina.

Stage 3

• The third stage begins immediately after delivery of the fetus.
• Continued uterine contractions separate the placenta from the uterine wall.
• The placenta, fetal membranes, and umbilical cord are expelled.
• Effective uterine contraction after placental delivery helps minimize postpartum blood loss.

Mechanism of Parturition

• Parturition is accomplished by coordinated, forceful contractions of the uterine myometrium.
• During the second half of pregnancy, intermittent painless contractions, often called Braxton Hicks contractions, may occur.
• These contractions increase gradually in frequency but do not produce cervical dilatation.
• True labor begins when regular uterine contractions become strong enough to cause progressive cervical change.
• In most pregnancies, the fetal head presents first and gradually descends into the pelvis during the final weeks of gestation.
• Near the onset of labor, the presenting part applies pressure to the cervix and lower uterine segment.
• Rupture of the fetal membranes may occur before or during labor, allowing release of amniotic fluid.
• Labor contractions usually originate near the uterine fundus and spread downward toward the cervix.
• Initially, contractions occur at intervals of approximately 10–15 minutes.
• As labor progresses, they become more frequent, longer in duration, and greater in intensity.
• Repeated contractions cause progressive cervical effacement and dilatation.
• Complete cervical dilatation reaches approximately 10 centimeters, permitting passage of the fetus through the birth canal.
• With each contraction, the fetus descends further through the cervix and vagina.
• During the second stage of labor, strong uterine contractions are assisted by voluntary contraction of the abdominal muscles.
• Increased intra-abdominal pressure enhances fetal expulsion.
• Continued coordinated contractions eventually result in delivery of the fetus.
• Following birth, persistent uterine contractions separate the placenta from the uterine wall.
• The placenta, fetal membranes, and umbilical cord are then expelled during the third stage of labor.
• These contractions also compress uterine blood vessels and help reduce postpartum blood loss.

Parturition Reflex

• Parturition is regulated by a neuroendocrine reflex involving the uterus, cervix, hypothalamus, and posterior pituitary gland.
• Stretch of the uterine wall by the growing fetus contributes to increased myometrial activity near term.
• During late pregnancy, the uterus produces increased amounts of prostaglandins, particularly prostaglandin E₂ and prostaglandin F₂α.
• These substances promote cervical ripening and enhance uterine contractions.
• Oxytocin further strengthens myometrial contractions and stimulates additional prostaglandin synthesis.
• Rising estrogen activity increases uterine excitability and enhances coordination of contractions.
• Estrogen also increases the number and sensitivity of oxytocin receptors in the myometrium.

Role of Oxytocin

• As the fetal presenting part descends, it stretches the cervix and lower uterine segment.
• Stretch receptors in these regions generate sensory impulses that travel to the hypothalamus through neural pathways.
• Hypothalamic stimulation triggers release of oxytocin from the posterior pituitary gland.
• Oxytocin increases the frequency and strength of uterine contractions.
• Stronger contractions cause greater cervical stretching and further stimulation of stretch receptors. This creates a positive feedback mechanism, often referred to as the Ferguson reflex.
• The cycle continues until delivery of the fetus is completed.
• Because the afferent component is neural and the efferent component involves hormonal release, parturition is considered a neurohumoral reflex.
• Coordinated action of oxytocin, prostaglandins, and uterine stretch is essential for successful labor and delivery.