Disappearance of para-amino-hippurate from amniotic fluid.

Eight pregnant women and three pregnant sheep received 400 mg of para-amino-hippurate (PAH) intraaminotically. Serial samples of amniotic fluid and maternal blood were obtained. In sheep samples of fetal blood were also withdrawn. PAH appeared in maternal plasma in all the cases. In all pregnant women PAH disappeared slowly from amniotic fluid (50% in 4 hours). In one ewe the study was performed as in humans and showed the same pattern of disappearance. In the other two, fetal urine was drained outside the amniotic fluid and PAH disappeared from it at a much faster rate (90% in 4 hours). PAH concentration in fetal urine was 100 times higher than in fetal plasma. Our findings in pregnant women seem to suggest that PAH disappears from the amniotic sac by a diffusion mechanism. On the other hand the results found in sheep also suggest that the fetus may have an active role in PAH concentration in amniotic fluid, eliminating part of the substance into maternal blood across the placenta but returning a major portion to the amniotic fluid with fetal urine.