The prenatal development period covers the time from conception to birth and is sometimes described in terms of trimesters (first, second, and third) or of three stages (germinal, embryonic, and fetal).
Conception. Conception, which occurs when the father's sperm cell penetrates the mother's ovum (egg), marks the beginning of development. The sperm cell/ovum combination creates a zygote, a one‐celled organism. All other cells in the body develop from this single cell. Each sperm and each egg cell carry 23 chromosomes, threadlike chains of DNA (deoxyribonucleic acid) that carry genetic information, which unite during fertilization to form 23 pairs of chromosomes. Genes are DNA segments that are functional units in hereditary transmission. After conception, all body cells except gametes (eggs or sperm) contain 23 pairs of chromosomes. The gender of the offspring is determined by the type of sex chromosome in the sperm that fertilizes the ovum; if it is Y bearing, the offspring will be male, and if it is X bearing, the offspring will be female.
The germinal stage. The germinal stage extends from conception to two weeks. During this period, the cells in the zygote divide rapidly, and the mass of cells moves slowly along the mother's fallopian tube to the uterus, where it is implanted in the uterine lining. During the implantation process, the placenta is formed. The placenta is a structure that serves as a life‐support system for the fetus, allowing oxygen and nutrients to pass into the fetus and waste products to pass out.
The embryonic stage. The embryonic stage begins after the cell mass is implanted in the uterus and lasts from two weeks through week eight. Most of the vital organs and body systems form at this time.
The fetal stage. The fetal stage is the third stage of prenatal development and covers the period from the end of week eight to birth. Cells continue to divide, body structures become functional, and the fetus becomes capable of movement. When a fetus is from 22 to 26 weeks old, it may survive if birth occurs, but chances for survival increase the closer the term is to 36 weeks.
Prenatal risks. Risks during the prenatal period include the following.
Adequate nutrition for the mother is imperative. Maternal malnutrition may affect not only the size of the infant but also the development of systems such as the immune system or the brain.
Teratogens are agents or substances that can produce developmental malformations. Many environmental pollutants (some pesticides, for example), as well as drugs, both therapeutic and recreational (for example, alcohol, cocaine, marijuana, and nicotine), can damage a developing fetus. Drugs such as marijuana remain in the fat cells of the body for as long as 30 days, and other drugs can enter the semen. Consequently, if a father or mother takes drugs prior to conception or if a mother takes drugs at any time during gestation, the fetus may be seriously damaged. A mother's ingestion of alcohol during pregnancy can produce fetal alcohol syndrome, a complex of birth defects including retardation, lower birth weight, and distinct facial features—a flat nose, wide‐set eyes, lack of an indentation on the upper lip (philtrum), and a thin upper lip. Ingestion of cocaine (or crack) during this period may produce a cocaine‐exposed infant, one who is hyperactive, so sensitive to environmental stimulation that it can't tolerate being held, and possibly retarded. Tobacco use during pregnancy and passive ingestion of smoke from other smokers can also damage the fetus and increase the risk for miscarriage and birth complications as can use of amphetamines, barbiturates, and some tranquilizers.