Fetal swallowing as a protective mechanism against oligohydramnios and polyhydramnios in late gestation sheep

Our objectives were to (1) quantify the relationship between daily swallowed volume and amniotic fluid volume (AF volume) in late gestation ovine fetuses and (2) use the resulting regression equation to explore the role of swallowing in regulating AF volume. Daily swallowed volume ranged from 36 to 1963 mL/d while experimental AF volume ranged from 160 to 6150 mL (n = 115). Swallowed volume was near zero when AF volume was far below normal, a maximum of 635 ± 41 (standard error) mL/d when AF volume was 1682 ± 31 mL and did not increase further with higher AF volumes. Computer simulations predicted that fetal swallowing would (1) return AF volume to normal in 5 to 6 days following an acute volume change in the absence of changes in other amniotic inflows or outflows and (2) stabilize AF volume in 4 to 8 days following sustained alterations in amniotic inflows or outflows other than swallowing.

The volume of AF swallowed each day by the fetus is a strong function of AF volume and reaches a maximum when mild polyhydramnios develops. With deviations in AF volume from normal, changes in fetal swallowing protect against oligohydramnios and polyhydramnios because the changes in swallowing over time reduce the extent of the AF volume change. However, with experimental changes in AF volume stabilizing in 1 to 2 days, it appears that swallowing is not the major regulator of AF volume.