25.10: Numerous strategies can help prevent fertilization.

Pregnancy depends on the occurrence of a great many events, relating to the production of sperm and eggs, fertilization, and implantation of the zygote in the uterus. Contraception, or birth control, is the attempt to prevent pregnancy. Although a wide variety of contraception strategies are used—with varying degrees of effectiveness—the methods can be grouped into five categories: (1) barrier methods, (2) hormonal methods, (3) intrauterine devices (IUDs), (4) “natural” methods, and (5) sterilization. FIGURE 25-20 summarizes the most commonly used methods of contraception and how they work.

Barrier Methods Barrier methods physically prevent conception by keeping sperm from coming in contact with an egg. They include the diaphragm, cervical cap, and both male and female condoms.

Diaphragm or cervical cap. A diaphragm or cervical cap—a dome-shaped piece of rubber placed in the vagina, blocking the cervix—prevents pregnancy by blocking sperm from reaching an egg. The use of spermicidal creams with a diaphragm or cervical cap (and with a condom as well) can increase its effectiveness.

Condoms. The condom—a thin rubber or natural membrane sheath placed on the penis or inside the vagina, covering the cervix—is another method of preventing contact between sperm and egg. Condoms have the added benefit of being one of the only methods of contraception that offer effective protection against sexually transmitted diseases, including HIV/AIDS. (See the next section for further discussion of this topic.)

Hormonal Methods Hormonal methods alter the levels of female hormones that influence the development and release of an egg. They may also change cervical mucus, reducing sperm function or transport. They include birth control pills, hormone injections, patches, or implants, and emergency contraceptive pills (or “morning-after” pills).

Birth control pills. Available since 1960, “the pill” is among the most effective of all methods of contraception, with a failure rate of 1% to 8%; that is, among 100 women using birth control pills over the course of one year, there would be 1–8 pregnancies. (If taken every day at exactly the same time, the pills are more than 99% effective; the problem is that not all women are consistent in taking them.) Among 100 women using no contraception, there would be about 85 pregnancies during one year.

Two chief varieties of birth control pills, or oral contraceptives, are available, one that contains synthetic versions of estrogen and progesterone, and one that contains only a synthetic progesterone. The estrogen-progesterone pill prevents ovulation by keeping estrogen levels just high enough that the release of FSH by the pituitary gland is never triggered. As long as FSH is never released, eggs do not develop and ovulation does not occur. The progesterone component of the pill causes just enough development of the lining of the uterus that a plug of mucus forms at the connection between the vagina and uterus, blocking sperm from getting through.

Taking the pill at the same time every day is essential. If more than 24 hours go by between pills, the estrogen level in the body begins to drop; if it gets below a critical level, FSH release by the pituitary is triggered, which can lead to ovulation and the risk of pregnancy.

The long-term health consequences of using birth control pills include a slightly elevated risk of cardiovascular disease, particularly among women who smoke. However, because birth control users have a reduced risk of pregnancy—a condition that carries with it some significant health risks—and a reduced risk of ovarian and endometrial cancers, women’s overall mortality risk is reduced while taking the pill (see Section 24-10).

Although the birth control pill is one of the most commonly used contraceptives, recent research has revealed that its hormones may alter women’s attraction to men. When not taking birth control pills, most women prefer the odor of men who have certain genetic combinations that are most different from their own. When taking the pill, however, their preference switches to the odor of men with genetic combinations most similar to their own. Much additional research in this area is under way.

Hormone injections or implants. Injections or capsule implants, just under the skin, of synthetic estrogen and progesterone, or progesterone only, represent a slight variation on birth control pills. The prevention of pregnancy works in the same way, but with the added convenience of not having to take a pill every day. Some implants, in fact, need to be replaced only once every three years. These methods are slightly more effective than birth control pills, because much of the user error is eliminated.

Emergency contraceptive pills. Not recommended as a long-term strategy for contraception, emergency contraceptive pills are available in several different types, containing different hormones or combinations of hormones, including progestin (a synthetic form of progesterone) and/or estrogen. Sometimes called “morning-after pills,” and marketed under several different names (including Plan B and ella), emergency contraceptive pills can be effective when taken up to five days after sex. They have a failure rate of up to 25% and are recommended only for cases of rape and for emergencies in which other methods of contraception have failed.

Intrauterine Devices (IUDs) An IUD is a small, T-shaped plastic or metal device inserted by a doctor into the uterus (FIGURE 25-21). IUDs change the conditions in the cervix and uterus to prevent pregnancy and/or inhibit the transit of sperm from the cervix to the Fallopian tubes. An IUD may contain copper or hormones (a type of progestin) and can be left in the uterus for three or four years. IUDs are very effective at preventing pregnancy and have a failure rate of just under 1%. Some women experience side effects from IUDs (including cramps and other pain) and may need to have the device removed.

“Natural” Methods “Natural” methods of contraception describe those in which individuals refrain from sexual intercourse completely or on days when fertility is likely, based on analysis of the woman’s menstrual cycle pattern, cervical secretions, and core body temperature.

Abstinence—not having sexual intercourse—is the most effective method of avoiding pregnancy, if practiced continuously. But, in reality, this is difficult to maintain, particularly in the context of a long-term relationship. Temporary abstinence during the times when conception is most likely (often called the “rhythm method”), while theoretically effective, has a failure rate as high as 25%, probably due to variation in the time of ovulation and the longevity of sperm in the female reproductive system (72 hours or more).

Sterilization Sterilization is the permanent alteration of the reproductive system to prevent the release of sperm or the movement of eggs down the Fallopian tubes. In a tubal ligation, the woman’s oviducts are cut and tied so that eggs cannot reach the uterus. In a vasectomy, the man’s vas deferens on each side is cut and tied so that sperm cannot reach the urethra, thereby causing the semen to carry no sperm. There are no side effects in either case. Each of these procedures, however, should be considered permanent (although in rare cases, the procedure can be successfully reversed).

In contrast to the contraception methods described above, the drug RU486—a progesterone receptor antagonist—is used to induce abortion in the first seven weeks of pregnancy. Considered an anti-hormone drug, RU486 blocks progesterone receptors in the uterus, thereby causing the lining of the uterus to be sloughed off and ending a pregnancy. In addition to causing abdominal pain and cramping, RU486 can also have other adverse effects, including nausea, vomiting, and fever.