Pre-delivery changes in amniotic fluid volume and composition in sheep

OBJECTIVE: To determine the changes in amniotic fluid (AF) volume and solute concentrations during the last 3 weeks of gestation in fetal sheep with a ligated urachus. METHODS: AF volume as well as solute concentrations of AF, fetal urine, fetal blood, and maternal blood were measured serially over the last 24 days of gestation in chronically catheterized fetal sheep. Statistical analyses included regression analysis and analysis of variance (ANOVA). RESULTS: Delivery occurred at 145.5 ± 1.1 (SE) days (n = 11 animals). AF volume averaged 913 ± 46 mL (n = 119 determinations total from 11 animals) and was unchanged with time when analyzed against either gestational age (P = .9) or time prior to delivery (P = .81). Amniotic osmolality, sodium, chloride, glucose, and calcium concentrations decreased as gestation progressed, while potassium and lactate concentrations increased. Only amniotic lactate underwent a pre-delivery increase in concentration. From multivariate regression, AF solute concentrations correlated positively with fetal blood and urine concentrations and negatively with gestational age. CONCLUSIONS: In fetal sheep with a ligated urachus, AF volume does not decrease prior to labor and delivery. This differs from the sharp pre-delivery decrease in AF volume that occurs in rats, mice, and pigs and is similar to the relatively constant AF volume in humans, baboons, and monkeys. Further, pre-delivery changes in AF lactate concentration may reflect the consequences of a low AF volume. Regression analysis suggests that fetal blood and urine independently contribute to AF solute concentrations and that gestational age-specific changes in intramembranous transport also may contribute.