Accumulation of maternal tissue (i.e., blood, uterine, and breast tissues and tissue stores) occurs primarily during the second trimester as does growth of the placenta, while growth of the fetus and increases in the amount of amniotic fluid occur most rapidly in the third trimester. Of an average 11 kg of total weight gain, the maternal compartment represents about 6 kg and the conceptus (fetus, placenta, and amniotic fluid) represents about 5 kg.
The composition of the weight gain also varies, but an 11-kg gain includes, on the average, about 7 kg of water, 3 kg of fat, and nearly 1 kg of protein (Hytten, 1980b). Of the total 7 kg of water gain, about 2–3 kg consist of maternal extracellular fluid. Pregnant women with edema, particularly generalized edema, may gain substantially more water. Sodium and other minerals also accumulate during pregnancy, but their direct contribution to the total weight gain is minimal.
Fat metabolism is affected substantially by pregnancy. Starting early in human pregnancy, women begin to store large amounts of fat, with sustained fat deposition occurring through the first two trimesters. Fat accumulation averages 3 kg, or approximately 15–25 percent of the prepregnant fat content (Hytten, 1980a). The rapid rate of maternal fat deposition occurs in many species (Widdowson and Spray, 1950), including the rabbit (Elphick et al., 1978), guinea pig, and sheep (Vernon et al., 1981), and the maternal total