Most of the normal weight gain in pregnancy is attributable to the uterus and its contents, the breasts, and expanded blood and extravascular extracellular fluid volumes. A smaller fraction results from metabolic alterations that promote accumulation of cellular water, fat, and protein, which are so-called maternal reserves. The average weight gain during pregnancy approximates 12.5 kg or 27.5 Ib, and this value has remained consistent across studies and over time (Hytten, 1991; Jebeile, 2016). Weight gain is considered in urther detail in Table 4-2 and in Chapter 9 (p. 165).
In pregnancy, greater water retention is normal and mediated in part by a drop in plasma osmolality of 10 mOsm/kg. This decline develops in early pregnancy and is induced by a reset of osmotic thresholds for thirst and vasopressin secretion (Fig. 4-3)
TABLE 4-2. Weight Gain Based on Pregnancy-Related Components
Total 650 4000 8500 12,500
Modified from Hytten, 1991l.
Weeks of pregnancy
FIGURE 4-3 Mean values (black line) ± standard deviations (blue lines) for plasma osmolality (Posm) measured at weekly intervals in nine women from preconception to 16 weeks. LMP = last menstrual period; MP = menstrual period. (Redrawn with permission from Davison JM, Dunlop W: Renal hemodynamics and tubular function in normal human pregnancy. Kidney Int 18:1o52, 1980.) (Davison, 1981; Lindheimer, 2001). Relaxin and other hormones are thought to playia role (Conrad, 2013).
At term, the water content of the fetus, placenta, and amnionic luid approximates 3.5 L. Another 3.0 L accumulates from expanded maternal blood volume and from uterus and breast growth. Thus, the minimum amount of extra water that the average woman accrues during normal pregnancy approximates 6.5 L. This corresponds to 14.3 lb.
Clearly demonstrable pitting edema of the ankles and legs is seen in most pregnant women, especially at the end of the day. This fluid accumulation, which may amount to a liter or so, results from greater venous pressure below the level of the uterus as a consequence of partial vena cava occlusion. A decline in interstitial colloid osmotic pressure induced by normal pregnancy also favors edema late in pregnancy (0ian, 1985).
Longitudinal studies of body composition show a progressive accumulation of total body water and fat mass during pregnancy. These two components as well as initial maternal weight and weight gained during pregnancy are highly associated with neonatal birthweight (Lederman, 1999; MardonesSantander, 1998). "Over-nourished" women are more likely to deliver oversized neonates, even when glucose tolerant (Di Benedetto, 2012).
The products of conception, the uterus, and maternal blood are relatively rich in protein rather than fat or carbohydrate. At term, the normally grown fetus and placenta together weigh about 4 kg and contain approximately 500 g of protein, or about half of the total pregnancy increase. The remaining 500 g is added to the uterus as contractile protein, to the breasts primarily in the glands, and to maternal blood as hemoglobin and plasma proteins.