| acquired immune deficiency syndrome (AIDS) | Viral disease that undermines effective functioning of the immune system. 99
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| cephalocaudal principle | Principle that development proceeds in a head-to-tail direction; that is, that upper parts of the body develop before lower parts of the trunk. 89
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| embryonic stage | Second stage of prenatal development (2 to 8 weeks), characterized by rapid growth and development of major body systems and organs. 90
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| fetal alcohol syndrome (FAS) | Combination of mental, motor, and developmental abnormalities affecting the offspring of some women who drink heavily during pregnancy. 96
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| fetal stage | Final stage of prenatal development (from 8 weeks to birth), characterized by increased differentiation of body parts and greatly enlarged body size. 91
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| germinal stage | First 2 weeks of prenatal development, characterized by rapid cell division, increasing complexity and differentiation, and implantation in the wall of the uterus. 89
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| gestation | Period of development between conception and birth. 86
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| gestational age | Age of an unborn baby, usually dated from the first day of an expectant mother's last menstrual cycle. 86
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| proximodistal principle | Principle that development proceeds from within to without; that is, that parts of the body near the center develop before the extremities. 89
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| spontaneous abortion | Natural expulsion from the uterus of a embryo that cannot survive outside the womb; also called miscarriage. 90
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| teratogen | Environmental agent, such as a virus, a drug, or radiation, that can interfere with normal prenatal development and cause developmental abnormalities. 93
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| ultrasound | Prenatal medical procedure using high-frequency sound waves to detect the outline of a fetus and its movements, used to determine whether a pregnancy is progressing normally. 91
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