Fetal rehydration via intraamniotic fluid: contribution of fetal swallowing.

Amniotic fluid volume is regulated by a balance of fetal fluid production and resorption. Although fetal swallowing is believed to be a major site of fluid resorption, additional routes of fluid exchange also may contribute. In our present study, five chronically prepared, water-restricted, pregnant ewes with singleton fetuses (128 +/- 1 d) were rehydrated via an intraamniotic infusion (100 mL/h over 90 min) of 0.075 M saline. In response to the maternal water restriction, significant increases were noted in maternal and fetal plasma osmolalities (306.6 +/- 1.2 to 315.4 +/- 2.4; 300.5 +/- 1.5 to 311.0 +/- 1.6 mosmol/kg, respectively) and arginine vasopressin concentrations (1.9 +/- 0.2 to 22.6 +/- 5.0; 1.5 +/- 0.1 to 8.5 +/- 2.2 pg/mL, respectively). After the intraamniotic infusion, fetal plasma osmolality (311.0 +/- 1.6 to 303.0 +/- 1.2 mosmol/kg) and hematocrit (36.7 +/- 1.9 to 33.8 +/- 1.4%) significantly decreased although there was no change in maternal arterial blood values. Fetal swallowing averaged 0.39 +/- 0.10 mL/min during the basal period and 0.34 +/- 0.17 mL/min at maximum dehydration, and decreased significantly to 0.19 +/- 0.07 mL/min in response to the intraamniotic infusion. These results indicate the rapid absorption of intraamniotic fluid by the dehydrated ovine fetus, despite the suppression of fetal swallowing. The volume swallowed during and after the intraamniotic infusion was insufficient to account for the observed changes in fetal plasma osmolality and hematocrit. Thus, alternative routes of fluid absorption (i.e. intramembranous flow) likely predominate under conditions of increased fetal plasma to amniotic fluid osmotic gradients.