Arginine vasopressin in primigravid human pregnancy. A prospective study.

The relationship between plasma osmolality (Posm) and plasma arginine vasopressin (Pavp) was determined in: (1) 62 normal primigravidas (P) at various stages of gestation and 35 nonpregnant, healthy volunteers (NP) under conditions of normal hydration or water deprivation, and (2) a separate group of 29 normotensive primigravidas during both the second and third trimesters under normal hydration. Pavp was similar in the P and NP groups, and Posm and Pavp were correlated significantly in both groups, but the osmotic threshold was lowered by 11 mosm/kg in primigravidas. In the prospective study, Pavp and Posm were correlated significantly at both stages. Pavp rose between trimesters, as did plasma volume and plasma renin activity, but urine volume and osmolality and Posm did not change significantly. These data confirm that resetting of the osmostat is operative in primigravid pregnancy and extend these observations by demonstrating that this phenomenon is established at least as early as mid-pregnancy. Furthermore, the osmoregulatory system does not appear to be altered by changes in plasma volume or plasma renin as pregnancy progresses.