74. Physiology of Contraception

Introduction

• Contraception encompasses a range of methods designed to prevent pregnancy by interfering with ovulation, fertilization, or implantation. Understanding the physiological basis, effectiveness, advantages, and limitations of different contraceptive methods is essential for informed reproductive health decisions and family planning.

• ·  Population stabilization remains an important public health objective in many countries, including India.
• ·  National population policies promote informed family planning and reproductive health services.
• ·  Contraceptive methods prevent pregnancy by inhibiting ovulation, fertilization, or implantation, depending on the method used.
• ·  Contraception is generally defined as prevention of pregnancy before establishment of a clinically recognized pregnancy.
• ·  Abortifacient methods terminate an established pregnancy after implantation has occurred.
• ·  Understanding the mechanism of action of different methods is essential for appropriate contraceptive counseling and use.

Classification

• ·  Contraceptive methods are classified as barrier methods (physical and chemical), intrauterine devices, hormonal methods, post-fertilization methods, and permanent sterilization procedures.
• ·  They may also be grouped as temporary or permanent methods based on reversibility and duration of action.

Physical Methods

In Males

• ·  The male condom is the most widely used physical barrier contraceptive.
• ·  It prevents sperm from entering the female reproductive tract.
• ·  Condoms also reduce the risk of many sexually transmitted infections.

In Females

• ·  The diaphragm is a vaginal barrier device that covers the cervix and prevents sperm entry into the uterus.
• ·  It is commonly used with spermicidal agents to improve effectiveness.
• ·  The vaginal sponge is another barrier method that acts by blocking and immobilizing sperm.

Chemical Methods

• ·  Spermicidal agents such as foams, creams, gels, and suppositories are applied intravaginally before intercourse.
• ·  These substances immobilize or destroy sperm, thereby reducing the likelihood of fertilization.
• ·  When used alone, spermicides have a relatively high failure rate and are often combined with barrier contraceptive methods to improve effectiveness.

Intrauterine Devices

• ·  Intrauterine devices (IUDs) are highly effective, long-acting reversible contraceptive methods placed within the uterine cavity.
• ·  They are particularly suitable for women who desire prolonged contraception.

Types

• ·  First-generation IUDs: Inert devices, such as the Lippes Loop, which are now rarely used.
• ·  Second-generation IUDs: Copper-bearing devices, including Copper T variants, which enhance contraceptive efficacy through continuous copper release.
• ·  Third-generation IUDs: Hormone-releasing devices containing levonorgestrel, which provide long-term contraception by local progestin release.
• ·  Modern copper and levonorgestrel-releasing IUDs are the most commonly used intrauterine contraceptive devices today.

Mechanism of Action

• ·  Intrauterine devices prevent pregnancy through multiple local effects within the female reproductive tract.
• ·  They primarily act before pregnancy is established by interfering with fertilization and by creating conditions unfavorable for implantation.
• ·  The presence of an intrauterine device induces a localized inflammatory response in the endometrium.
• ·  This reaction alters the uterine environment and reduces the viability of sperm and ova.
• ·  Consequently, the likelihood of fertilization is significantly decreased.
• ·  Copper-containing devices release copper ions into the uterine cavity.
• ·  Copper enhances the local inflammatory response and exerts toxic effects on sperm.
• ·  It reduces sperm motility, impairs sperm transport, and interferes with sperm capacitation.
• ·  These effects markedly reduce the probability of successful fertilization.
• ·  Levonorgestrel-releasing devices continuously release a progestin into the uterine cavity.
• ·  The hormone increases the viscosity of cervical mucus, making sperm penetration difficult.
• ·  It also suppresses endometrial proliferation, rendering the endometrium less receptive to implantation.
• ·  In some users, hormonal devices may also inhibit ovulation intermittently.
• ·  Owing to these combined actions, modern intrauterine devices are among the most effective reversible contraceptive methods available.

Merits and Demerits

• Merits and demerits of IUDs are summarized in Table 74.1.

Hormonal Contraceptives

• ·  Hormonal contraceptives include oral contraceptive pills, injectable preparations, subcutaneous implants, and vaginal rings.
• ·  Oral contraceptives may be combined estrogen–progestin pills, progestin-only pills, or emergency contraceptive pills.

Oral Contraceptive Pills (OCPs)

• ·  Combined oral contraceptives contain low doses of estrogen and a progestin.
• ·  They are usually taken for 21 consecutive days in each menstrual cycle, followed by a hormone-free interval.
• ·  Their primary action is suppression of ovulation through inhibition of luteinizing hormone and follicle-stimulating hormone secretion.
• ·  Progestin also thickens cervical mucus, reducing sperm penetration into the uterus.
• ·  In addition, hormonal contraceptives alter endometrial development, making the uterine lining less receptive to implantation.
• ·  These combined mechanisms provide highly effective and reversible contraception when used correctly.

Side Effects of OCP

• ·  Modern oral contraceptive pills are highly effective when used correctly, but no contraceptive method is completely failure-free.
• ·  Adverse effects vary with hormone dose, duration of use, age, and underlying health conditions.

Cardiovascular Side Effects

• Combined hormonal contraceptives may increase the risk of venous thromboembolism, stroke, and myocardial infarction in susceptible individuals.
• The risk is greater in women older than 35 years who smoke.
• Mild elevation of blood pressure may occur because estrogen can enhance activity of the renin–angiotensin–aldosterone system.

Carcinogenesis

• ·  Current evidence indicates a slight increase in the risk of cervical cancer with prolonged use in some women.
• ·  Rare benign hepatic tumors have been reported.
• ·  The relationship between oral contraceptive use and breast cancer remains complex and requires individual risk assessment.

Metabolic side Effects

• ·  Hormonal contraceptives may alter lipid metabolism and coagulation profiles.
• ·  Some users may develop mild insulin resistance or impaired glucose tolerance.

Miscellaneous

• ·  Additional adverse effects include breast tenderness, nausea, headache, migraine, weight changes, and cholestatic jaundice.
• ·  Most side effects are mild and improve with appropriate contraceptive selection and medical supervision.

Depots

• ·  Long-acting progestin-only contraceptives are available as subcutaneous implants, injectable preparations, and vaginal rings.
• ·  Subcutaneous implants release progestin gradually and provide effective contraception for several years.
• ·  Injectable preparations, such as depot medroxyprogesterone acetate, are administered at regular intervals, usually every three months.
• ·  Hormone-releasing vaginal rings provide sustained local delivery of contraceptive hormones.

Post-Conceptional Pills

• ·  Emergency contraceptive methods may be used after unprotected intercourse to reduce the risk of pregnancy.
• ·  They are most effective when taken as soon as possible, preferably within 72 hours.
• ·  These methods primarily act by delaying or inhibiting ovulation and interfering with fertilization.
• ·  Levonorgestrel-containing preparations are commonly used emergency contraceptives.
• ·  Mifepristone, a progesterone receptor antagonist, is also effective for emergency contraception.
• ·  By blocking progesterone action, mifepristone alters endometrial receptivity and may delay ovulation, thereby reducing the likelihood of pregnancy.

Other Methods

The Rhythm (Safe Period) Method

• The rhythm method is a natural family planning technique based on avoiding unprotected intercourse during the fertile period of the menstrual cycle.
• In a regular 28-day cycle, ovulation typically occurs around day 14, although the timing may vary among individuals and between cycles.
• Because sperm can remain viable in the female reproductive tract for up to 5 days and the ovum remains fertilizable for about 24 hours, the fertile window extends beyond the day of ovulation.
• Couples using this method avoid unprotected intercourse during the estimated fertile period to reduce the chance of conception.
• For women with variable cycle lengths, the fertile period can be estimated using previous menstrual records.
• The first fertile day is calculated by subtracting 18 days from the length of the shortest cycle.
• The last fertile day is calculated by subtracting 11 days from the length of the longest cycle.
• For example, if cycle lengths range from 25 to 32 days, the estimated fertile period extends from day 7 to day 21.
• The method requires careful cycle tracking, regular record keeping, and good understanding of reproductive physiology.
• Variations in ovulation timing due to stress, illness, hormonal changes, or irregular cycles can reduce its reliability.
• Fertility awareness methods are more effective when combined with additional indicators such as basal body temperature and cervical mucus observations.
• Although inexpensive and free from drug-related adverse effects, the rhythm method has a higher failure rate than most modern contraceptive methods.
• No phase of the menstrual cycle can be considered completely risk-free for pregnancy because ovulation may occasionally occur earlier or later than expected.

• Contraceptive failure rates are generally lower when methods are used correctly and consistently according to recommended guidelines.
• Proper use significantly improves the effectiveness of both temporary and long-acting contraceptive methods.

Coitus Interruptus

• In the withdrawal method, the penis is withdrawn from the vagina before ejaculation.
• This aims to prevent deposition of sperm within the female reproductive tract and thereby reduce the likelihood of fertilization.
• The method has a relatively high failure rate because timing errors may occur.

Breast-feeding

• Exclusive breastfeeding can temporarily suppress ovulation through lactational amenorrhea.
• Elevated prolactin levels inhibit gonadotropin-releasing hormone secretion, reducing ovulation.
• This method is effective only under specific conditions and provides temporary contraception.

Family Planning Operations

• Sterilization is a highly effective permanent method of contraception for individuals who have completed their families. It may be performed in either the male or female partner.

Male Sterilization

• Male sterilization is achieved by vasectomy, in which both vas deferens are occluded to prevent sperm transport.
• The procedure does not affect testosterone secretion, libido, or erectile function.
• Although reversal procedures are possible, successful restoration of fertility is not guaranteed.

Female Sterilization

• Female sterilization is performed by bilateral tubal occlusion or tubal ligation.
• Blocking the uterine tubes prevents the sperm and ovum from meeting, thereby preventing fertilization.
• Reversal surgery may be attempted in selected cases, but outcomes vary.

Failure Rates of Contraceptive Methods

• Male and female sterilization are among the most effective contraceptive methods available.
• Their failure rates are very low compared with most temporary contraceptive methods. Therefore, sterilization is considered a reliable option for long-term family planning.