Reproduction Health Article

REPRODUCTION

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.