When stressed by dehydration or water loading, pregnant women respond in a manner similar to that of nonpregnant women, in spite of the large increase in glomerular filtration rate that accompanies gestation, which might increase filtered solute substantially and thus compromise the extremes of concentration and dilution.

The metabolism of arginine vasopressin is markedly altered during pregnancy as metabolic clearance rate increases fourfold between early and mid-gestation (Davison et al., 1989, 1993). This is due to the appearance of high circulating levels of placental vasopressinase (a cystine aminopeptidase). Normally the production rate of arginine vasopressin is sufficient to overcome the increased disposal rate, but there are rare instances of subclinical central diabetes insipidus that become apparent by the increased metabolic clearance rate of arginine vasopressin in pregnancy (Baylis et al., 1986; Lindheimer and Davison, 1995). There are also instances of overproduction of vasopressinase resulting in a syndrome-labeled transient diabetes insipidus during pregnancy (Durr et al., 1987; Lindheimer and Davison, 1995).

Summary. While there are differences in plasma osmolality during pregnancy, the differences are not a result of poor hydration status and are short term. Therefore, an AI for total water (drinking water, beverages, and food) during pregnancy is based on the estimated median total water intake during pregnancy (Appendix Table D-1). In the NHANES, water from food provided 22 percent of the estimated total water intake, slightly more than the 19 percent of the estimated total water consumption seen in nonpregnant women (Appendix Table D-4).