Water exchange between the pregnant ewe, the foetus and its amniotic and allantoic fluids

Water exchange between the ewe and its foetus was measured in two Merino ewes maintained with continuous feeding under thermoneutral conditions from about day 90 of gestation to term. Catheters were established in the maternal pulmonary artery (MPA), the foetal dorsal aorta (FDA) and the amniotic and allantoic sacs. Doses of tritiated water were given into either the MPA or the FDA on five occasions, three for one ewe and two for the other, at least 6 days apart and samples were taken from all catheters for 31 h following the dose. An open, eight-compartment restricted model was developed which simultaneously fitted the tracer data and the ewes' water balance, determined by measurement and calculation. The ewes delivered live lambs at term. Water was exchanged between ewe and foetus at 16-43 l h(-1) whereas net flow to the foetus averaged 82 ml day(-1). Turnover times were 20-39 min in the ewes' body water, 2-7 min in foetal body water, 10-58 min in amniotic water and 3-22 min in allantoic water; the whole-body half-times were 4.5-5.7 days. The data suggest that intramembranous exchange was the major contributor to water exchange in amniotic and allantoic fluids. The mean residence time of water in the rumen (39-52 min) was shorter than in non-pregnant sheep, suggesting that blood flow to the rumen increased during mid to late gestation.