Body Water. Weight increases about 12 kg during an average pregnancy, but approximately 15 percent of normal pregnant women also develop generalized swelling and additional weight gain (≈ 2.5 kg) (Chesley, 1978; Forsum et al., 1988; Hytten, 1980; Hytten and Leitch, 1971; Lindheimer and Katz, 1985). Most of this added weight is water and includes the products of conceptus and gains within the expanded maternal intra- and extracellular spaces.
Total body water has been measured during gestation with deuterium, the stable isotope of oxygen, or by bioelectric impedance (Catalano et al., 1995; Chesley, 1978; Forsum et al., 1988; Hytten, 1980; Hytten and Leitch, 1971; Lindheimer and Katz, 1985). Results vary (due partly to different methodologies, but also to the period of testing with interpolation from final measurement until term), with findings of total accumulation from 6 to 9 L, of which 1.8 to 2.5 L are intracellular fluid. The increases in maternal vascular and interstitial volumes are discussed in Chapter 6, and further discussions of the validity of methodologies utilized primarily in studies of the extracellular-extravascular compartment are discussed by Chesley (1978) and Lindheimer and Katz (1985, 2000).
Hydration Status and Plasma Osmolality. Plasma osmolality decreases by 8 to 10 mOsmol/kg during normal gestation. The decrement that normally starts during the luteal phase of the menstrual cycle continues through conception, reaching its lowest point during gestational week 10, after which the decline is sustained until term (Davison et al., 1981, 1984; Lindheimer and Davison, 1995). Since only approximately 1.5 mOsmol/kg of the decrease can be attributed to the small decrement in circulating urea, most of the decline is due to lower levels of sodium and its attendant anion. Thus gestation is characterized by a decrease in body tonicity (i.e., effective osmolality). The reason for this decline is a parallel decrease in the osmotic thresholds for arginine vasopressin release and thirst, with the pregnant woman then concentrating and diluting urine appropriately around this new steady-state body tonicity (Davison et al., 1981; Lindheimer and Davison, 1995). Since the threshold for arginine vasopressin release decreases a bit more rapidly than that for thirst, pregnant women may experience a short transient period of polyuria during early gestation (Davison et al., 1988).