Growth of the Fetus
The embryo becomes a fetus about 8 weeks after fertilization (figure 20.13). The beginning of the fetal period is marked by the beginning of bone ossification. In the embryo, most of the organ systems are developing, whereas in the fetus the organs are present. During the fetal period, the organ systems enlarge and mature. The fetus grows from about 3 cm and 2.5 g (0.09 oz) at 8 weeks to 50 cm and 3300 g (7 lb, 4 oz) at the end of preg-nancy. The growth during the fetal period represents more than a 15-fold increase in length and a 1400-fold increase in weight. The amniotic fluid contains toxic waste products from thefetus’s digestive tract and kidneys. Fine, soft hair called lanugo (lă-noo′ gō) covers the fetus, and a waxy coat of loose epithelial cells called vernix caseosa (ver′ niks kā′ sē-ō′ să) forms a protec-tive layer between the fetus and the amniotic fluid.
Subcutaneous adipose tissue that accumulates in the fetus provides a nutrient reserve, helps insulate, and aids the newborn in sucking by strengthening and supporting the cheeks, so that a small vacuum can be developed in the oral cavity.
Peak body growth occurs late in gestation, but as the placenta reaches its maximum size, the oxygen and nutrient supply to the fetus becomes limited. Growth of the placenta essentially stops at about 35 weeks, limiting fetal growth.
At approximately 38 weeks of development, the fetus is ready to be delivered. The average weight at this point is 3250 g (7 lb, 2 oz) for a female fetus and 3300 g (7 lb, 4 oz) for a male fetus.