Human gestation is divided into trimesters

In humans, gestation, or pregnancy, lasts about 266 days, or 9 months. Gestation is shorter in smaller mammals (21 days in mice) and longer in larger mammals (600 days in elephants). The events of human gestation (Figure 43.17) can be divided into three trimesters of roughly 3 months each. The first trimester (see Figure 43.17A) is a time of rapid cell division and tissue differentiation. Because signal transduction cascades that determine sequences of developmental processes are in their early stages, the first trimester is the period when the embryo is most sensitive to damage from radiation, drugs, chemicals, and pathogens that can cause birth defects. An embryo can be damaged before the mother even realizes she is pregnant. A classic case is that of thalidomide, a drug widely prescribed in Europe in the late 1950s to treat nausea. Women who took this drug in the fourth and fifth weeks of pregnancy, when the embryo’s limbs are beginning to form, gave birth to children with missing or severely malformed arms and legs.

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Figure 43.17 Stages of Human Development (A) At 4 weeks, most of the embryo’s organ systems have been formed and the heart is beating. The body structures of this 8-week-old embryo are forming rapidly. (B) At 4 months, the fetus has fully formed limbs with fingers and toes and moves freely within the amniotic cavity. (C) This fetus is well along in its ninth month. Soon its lungs will be mature enough to trigger the onset of contractions and birth.

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During the second trimester the fetus grows rapidly; its limbs elongate, and its fingers, toes, and facial features become well formed (see Figure 43.17B). Eyebrows and fingernails grow, and the fetus’s nervous system develops rapidly. Fetal movements are first felt by the mother early in the second trimester and become progressively stronger and more coordinated.

The fetus grows rapidly during the third trimester. As this final stage approaches its end (see Figure 43.17C), the internal organs mature. The digestive system begins to function, the liver stores glycogen, the kidneys produce urine, and the brain undergoes cycles of sleep and waking. A human infant is born when the last of its critical organs—the lungs—mature.

Although the first-trimester embryo is the most susceptible to adverse effects of drugs, chemicals, and diseases, the potential for serious effects from exposure to environmental factors exists throughout pregnancy. Protein malnutrition and exposure to alcohol and cigarette smoke are examples of factors that can result in low birth weight, mental retardation, and other developmental complications.