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a Number of pregnancies per 100 women per year a Number of pregnancies per 100 women per year tion). A female condom, which creates an impermeable lining of the vagina, is also available.

The diaphragm is a dome-shaped piece of rubber with a firm rim that fits over the woman's cervix and thus blocks sperm from entering the uterus. Smaller than the diaphragm is the cervical cap, which fits snugly just over the tip of the cervix. Both the diaphragm and the cervical cap are treated first with jelly or cream containing a spermicide—a chemical that kills or incapacitates sperm—and then inserted through the vagina before sexual intercourse. Annual failure rates are about 15 percent—the same as for condoms.

Spermicidal foams, jellies, and creams can be used alone by placing them in the vagina with special applicators. Used in this way, they have an annual failure rate of 25 percent or more. Douching (flushing the vagina with liquid) after intercourse, in spite of popular belief, is almost useless as a method of birth control. Sperm can reach the upper regions of the oviducts within 10 minutes after ejaculation.

The effectiveness of barrier methods can be greatly improved if different ones are used in combination. For example, if the man uses a condom and the woman a diaphragm, the failure rate is extremely low.

preventing ovulation. The widely used oral contraceptives, or birth control pills, work by preventing ovulation. Their mechanisms of action take advantage of the roles of estrogen and progesterone as negative feedback signals to the hypothalamus and the pituitary. The most common pills contain low doses of synthetic estrogens and progesterones (progestins). By keeping the circulating levels of gonadotropins low, these hormones interfere with the maturation of follicles and eggs, suspending the ovarian cycle. The uterine cycle is usually allowed to continue, however, by ceasing the pills every 21 to 23 days.

The negative side effects of oral contraceptives have been the topic of extensive discussion. These side effects include increased risk of blood clot formation, heart attack, stroke, and breast cancer. However, these side effects are associated mostly with pills containing higher hormone concentrations than are used in modern pills. For pills in use today, the risk of these side effects is low, except for women over 35 years old who smoke, for whom the risk is significantly greater. The risk of death from using "the pill" is less than the risk associated with a full-term pregnancy. The pill is the most effective method of contraception other than sterilization or perhaps combined barrier methods. Oral contraceptives have an annual failure rate of less than 1 percent.

The "mini-pill" is an oral contraceptive that contains very low doses of progestins. Although it may interfere with the normal maturation and release of eggs, its principal mode of action is to alter the environment of the female reproductive tract so that it is not hospitable to sperm. Cervical mucus normally becomes watery at the time of ovulation, but low levels of progestins keep the mucus thick and sticky so that it blocks passage of sperm.

Long-lasting injectable or implantable steroids are also used to block ovulation through negative feedback effects. Depo-Provera is an injectable progestin that blocks the release of gonadotropins for several months. Another device, called Norplant, consists of thin, flexible tubes filled with progestin. Several of these tubes are inserted under the skin, where they continue to release progestin slowly for years.

preventing implantation. A highly effective method of contraception (with a failure rate varying from 1 percent to about 7 percent) is the intrauterine device, or IUD. The IUD is a small piece of plastic or copper that is inserted into the uterus. The IUD probably works by causing an inflammatory response that includes the release of prostaglandins, which prevent implantation of the fertilized egg.

Another way of interfering with implantation is through the use of "morning-after pills," which deliver high doses of steroids, primarily estrogens. By acting in several ways on the oviducts and the endometrium, this treatment prevents implantation. Morning-after pills can be effective up to several days after sexual intercourse.

The drug RU-486, developed in France, is not a contraceptive pill, but a contragestational pill. It blocks progesterone receptors, thereby interfering with the normal action of progesterone produced by the corpus luteum, which is necessary for the maintenance of the endometrium in early pregnancy. If RU-486 is administered as a "morning-after pill," it prevents implantation. However, RU-486 can be effective even if taken at the time of the first missed menstrual period, after implantation has begun. After a few days of treatment with RU-486, the endometrium regresses and sloughs off, along with the embryo, which is in very early stages of development.

sterilization. One foolproof method of contraception is sterilization. Male sterilization by vasectomy is a simple operation (cutting and tying of the vasa deferentia) that can be performed under a local anesthetic in a doctor's office (Figure 43.16a). After this minor surgery, the semen no longer contains sperm. Sperm production continues, but since the sperm cannot move out of the testes, they are destroyed by macrophages. Vasectomy does not affect a man's hormone levels or his sexual responses, and even the amount of semen he ejaculates is essentially unchanged.

In female sterilization, the aim is to prevent the egg from traveling to the uterus and to block sperm from reaching the egg. The most common method is tubal ligation: cutting and tying of the oviducts (Figure 43.16b). Alternatively, the oviducts may be burned (cauterized) to seal them off. As in

(a) Vasectomy

A short piece of vas deferens is cut out...

(a) Vasectomy

A short piece of vas deferens is cut out...

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Testis Scrotum

(b) Tubal ligation

The oviducts are cut and the ends are folded over and tied off.

Oviduct

(b) Tubal ligation

The oviducts are cut and the ends are folded over and tied off.

Oviduct

Sterilization Techniques

Ligament 'Ovary

43.16 Sterilization Techniques (a) Vasectomy is the technique for male sterilization. (b) Tubal ligation is the sterilization procedure most commonly performed on human females.

Ligament 'Ovary

43.16 Sterilization Techniques (a) Vasectomy is the technique for male sterilization. (b) Tubal ligation is the sterilization procedure most commonly performed on human females.

the male, these procedures do not alter reproductive hormones or sexual responses.

abortion. Once a fertilized egg is successfully implanted in the uterus, any termination of the pregnancy is called an abortion. A spontaneous abortion is the medical term for what most people call a miscarriage. Miscarriages are common early in pregnancy (about 10%); most of them occur because of an abnormality in the fetus or in the process of implantation. Abortions that are not spontaneous, but are the result of medical intervention, may be performed either for therapeutic purposes or for fertility control. A therapeutic abortion may be necessary to protect the health of the mother, or it may be performed because prenatal testing reveals that the fetus has a severe defect. Of the approximately 3 million unintended pregnancies in the United States each year, almost half are ended by abortion.

In a medical abortion, the cervix is dilated and the fetus and the endometrium are removed from the uterus by physical means. When performed in the first trimester of a pregnancy, a medical abortion carries less risk of death to the mother than a full-term pregnancy. After the first 12 weeks of pregnancy, the risk rises, but even through the second trimester, it is less than that of a full-term pregnancy.

controlling male fertility. You may ask why all the pharmacological approaches to controlling fertility are applied to females. The control of male fertility is a difficult problem. First, spermatogenesis is a continuous rather than a cyclical event, and it is difficult to block a particular step in a continuous process. The ovarian cycle is more vulnerable to manipulation because certain events must happen at certain times and in a certain sequence for ovulation and implantation to occur. Second, the suppression of sper-matogenesis must be total to be effective, since technically it takes only a single sperm to fertilize an egg, and normally millions are produced continuously. Such suppression requires powerful and constant chemical intervention, with associated side effects.

Essentials of Human Physiology

Essentials of Human Physiology

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