Chapter 4 Embryo and fetus
By the seventh week after fertilization most of the organs have formed and the embryo becomes a fetus. The early growth of the fetus is shown in Table 4.1.
NUTRITION
Fetal growth is determined by many factors, both genetic and environmental. Of the latter, adequate placental perfusion and placental function are crucial. Maternal nutrition is not a limiting factor except in cases of extreme starvation, although chronic undernutrition may be associated with anaemia and may lead to a low-birthweight baby.
The fetus, insulated in its protective amniotic sac and relatively weightless, directs most of the energy supplied to it to growth. The energy is derived mainly from glucose. Only small amounts of lipids, as free fatty acids, cross the placenta until the fourth quarter of pregnancy. Any excess carbohydrate, after the growth and metabolic energy needs of the fetus have been met, is converted into lipids, and this conversion increases as term approaches.
From the 30th gestational week the fetal liver becomes increasingly efficient and converts glucose into glycogen, which is stored in the fetal heart muscle, the skeletal muscle and the placenta. Should fetal hypoxia occur the fetus is able to obtain energy from the heart muscle and placenta for anaerobic glycolysis (see Ch. 20).
The fat stores of an 800 g fetus (24–26 weeks’ gestation) constitute 1% of its body weight; by the 35th week fat constitutes 15% of fetal body weight.
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