Ovine fetal urine contribution to amniotic and allantoic compartments

The ovine pregnancy has been extensively studied as a model for amniotic fluid metabolism potentially serving as a model for human oligohydramnios. However, it is recognized that the ovine pregnancy contains an allantoic fluid compartment not present in human pregnancies at term. Earlier studies in sheep suggested that the fetal urine contribution to allantoic fluid diminished at term, but without an explanation for maintenance of the significant volume of the allantoic cavity. In the present study we examined the relative fetal urine excretion of 3H-inulin into the allantoic and amniotic cavities in the near-term ovine pregnancy. Amniotic and allantoic volumes, as determined by technetium-labelled dextran and chromium-tagged erythrocyte dilution, were similar although there were significant differences in composition. The allantoic fluid compartment received an equal or greater proportion of fetal urine than did the amniotic fluid during the study period. These results indicate the importance of the allantoic fluid volume and composition in the study of ovine fluid dynamics.