Reproduction is the process by which offspring are formed and genetic material is passed on from one generation to the next. In humans, reproduction is sexual.

FERTILIZATION

Gametes are produced by the reproductive glands, or gonads. Female gonads (ovaries) produce ova and male gonads (testes) produce sperm. Both ovum and sperm are haploid, which means that they contain half the normal (diploid) amount of genetic material (DNA) of the adult. During coitus, about 100 million sperm are deposited in the vagina, but only a few hundred reach the site of fertilization in the fallopian tube. One sperm penetrates the ovum during a process that leads to the fusion of the sperm and ovum nuclei, which contain the DNA. This fusion restores the diploid chromosome number, so that offspring inherit about half of their genes from each parent.

The fertilized ovum, now called the zygote, undergoes repeated cell divisions as it moves toward the uterus and implants in the endometrium. Only 20 to 25 percent of fertilized ova result in successful pregnancies. The rest fail to divide, fail to implant, or miscarry. Many of these unsuccessful pregnancies are genetically abnormal.

PREGNANCY

During the first trimester of pregnancy, the conceptus differentiates various specialized structures and organs, a process called embryogenesis. At the completion of this period, the embryo becomes a fetus. During the second and third trimesters, the fetus continues to grow and mature. By the ninth month, the fetus should be able to breathe on its own and maintain a normal body temperature. Survival rates are greater than 99 percent for babies born in most developed countries. Infant mortality is an important measure of public health and is influenced by many factors, including the proportion of births that occur prematurely or with birthweight that is too low. Other factors, such as the availability of services to ensure safe delivery and good health for mother and fetus, also influence infant mortality.

About 3 percent of infants have major congenital anomalies that are apparent in the first year of life. Such birth defects are the most frequent causes of infant mortality in many developed countries. Some congenital anomalies result from chromosomal abnormalities or mutations of single genes or gene pairs, but the cause of most birth defects is unknown. Many congenital anomalies appear to result from combinations of genetic and nongenetic factors that have not yet been identified.

Supplementation of the mother's diet with folic acid around the time of conception reduces the occurrence of neural tube defects and certain other birth defects. Reducing the occurrence of birth defects by folic acid dietary supplementation or food fortification is an important but largely unfulfilled public health opportunity.

Teratogenic exposures are thought to be responsible for about 10 percent of congenital anomalies. A variety of infections, medications, alcohol, and other agents can adversely affect embryonic or fetal development under certain exposure conditions. The embryo is most sensitive to damage from most teratogenic exposures between two and ten weeks after conception. Teratogenic exposures are an especially important cause of birth defects because they are potentially preventable.

CONTRACEPTION

Contraception is the process or means used to prevent pregnancy. Contraceptive options include abstinence, spermicide, male condoms, female condoms, hormonal methods, diaphragm, intra-uterine devices (IUDs), and surgical sterilization. Different methods of birth control have different degrees of effectiveness against pregnancy and of protection against sexually transmitted diseases (STDs). Each method has specific advantages, risks, and limitations.

The most effective method of birth control is abstinence. This method is 100 percent effective against pregnancy and has a decreased risk of contracting STDs. For sexually active people, the effectiveness for pregnancy prevention by surgical sterilization or hormonal contraceptive methods, when properly used, is about 99 percent. IUDs and condoms with spermicide can provide protection against pregnancy that is almost as good, although inconsistent use is often a limiting factor in practice with methods such as condoms. Condoms can also provide protection against STDs.

The use of effective contraceptive methods has led to fewer unwanted pregnancies and possibly a decrease in the spread of STDs in industrialized countries. Declining infant mortality has produced a growing population and a greater need for family planning measures in most of the world, but the overall reproductive health of women has often received less attention. This is evidenced by high rates of maternal mortality and STDs. It is estimated that about 600,000 maternal deaths occur each year, with the overwhelming majority in developing countries. Close to 80 percent of these deaths are direct results of complications rising during pregnancy, delivery, or the post-partum period. The remaining 20 percent are due to preexisting maternal conditions that worsen during pregnancy, such as HIV/AIDS (human immunodeficiency virus/acquired immunodeficiency syndrome), malaria, heart disease, or hepatitis. Maternal mortality is highest in south and southeast Asia, sub-Saharan Africa, and Latin America.

One possible consequence of STDs is pelvic inflammatory disease (PID), a major cause of damage to female reproductive organs that can lead to death if untreated. Barrier contraceptive methods that decrease the risk of contracting STDs may also protect against PID. Oral contraceptive use is also associated with a decreased risk of PID, although the mechanism of this protection is unknown. IUDs are believed to increase the risk of PID, especially in women who are at an increased risk for STDs. PID currently affects about 1 million American women, most of whom are from lower socioeconomic classes.

Since no method of birth control except for complete abstinence is 100 percent effective, unwanted pregnancies do occur. In these cases, induced abortion may be used to terminate the pregnancy. Induced abortion has important ethical, psychological, and medical drawbacks when used as a substitute for contraception. Physical complications are frequent when abortions are done without proper sterile technique or by individuals who lack the necessary training and skills. Complications may include severe pain, infection, uterine perforation, hemorrhage, and death. Unsafe abortion is a major cause of death and illness for women of childbearing age. It is estimated that complications of unsafe abortions are responsible for 13 percent of maternal deaths. In some parts of the world, one-third or more of all maternal deaths are associated with unsafe abortions.

INFERTILITY

Infertility is an inability to have children. In medical practice, infertility is diagnosed when a couple has been unable to conceive after one year of unprotected intercourse timed to coincide with ovulation. If the woman is over thirty-five years of age or has been unable to carry a pregnancy to term, this time is reduced to six months. Infertility affects 5.3 million Americans. Approximately 40 percent of infertility is due to female factors, 40 percent to male factors, and 20 percent to either combined or unknown factors. STDs and PID are two conditions that can lead to infertility in women. Therefore, educating the public about the risks of these infections is important in preventing infertility and improving women's health.

Conventional treatments for infertility depend on the cause and may include hormonal therapy and surgical procedures. In cases in which conventional methods fail, more advanced assisted reproductive technologies (ARTs) may be used. Current use of ARTs other than artificial insemination by donor is restricted because of limited availability, expense, and relatively low success rates.

PRENATAL DIAGNOSIS AND SCREENING

A variety of prenatal diagnostic techniques are available for couples at increased risk of having a child with certain genetic or developmental abnormalities. These tests include amniocentesis and chorionic villous sampling (CVS). Such invasive techniques are associated with small risks of inducing pregnancy loss or fetal damage and require skilled operators and sophisticated ultrasonography equipment. Ultrasound examination and maternal serum screening tests, which are not associated with any known fetal risks, are used for routine pregnancy screening in some jurisdictions. These techniques can identify many, but not all, fetuses with Down syndrome or serious structural abnormalities such as spina bifida. Because very few fetal abnormalities can be treated effectively before delivery, prenatal screening or diagnosis may raise serious ethical and social issues related to the abortion of fetuses considered to be less than perfect.

Increased availability and public support of reproductive medical care and related educational and prevention initiatives in most developed countries have had an important beneficial effect on the health of women and young children. However, such services are not readily available in all parts of the world, and maternal and infant mortality as well as death and illness from STDs are far too frequent. Providing public health interventions to deal with these problems in an appropriate cultural, social, and religious context remains an urgent and often very challenging priority.

JAN M. FRIEDMAN

ROXANA MOSLEHI

(SEE ALSO: Abortion Laws; Contraception; Maternal and Child Health; Population Growth; Population Policies; Pregnancy; Prenatal Care; Sexually Transmitted Diseases; Teratogens; Women's Health)