Creatinine exchange between mother, fetus, and amniotic fluid

Creatinine exchange between mother, fetus, and amniotic fluid was investigated in rhesus monkeys in late pregnancy in which a different radioactive-labeled form of creatinine was injected simultaneously into either of two of the three possible compartments. Creatinine specific activity was then determined for each isotope in each compartment over the ensuing 4-6 h. A rapid and extensive bidirectional exchange occurred between all three compartments. Maternal-fetal flux, while bidirectional, seemed to be predominately from mother to fetus. Approximately 35% of labeled creatinine originally injected into the fetus and 25% of labeled creatinine originally injected into the mother were present in the amniotic sac within 2 h. Creatinine pool size was calculated to be approximately 6 mg/kg in both mother and fetus. Comparison of experiments involving living had dead fetuses indicated that the route of creatinine transfer from mother to amniotic fluid was through the living fetus and directly across the chorioamnion in the dead fetus. In addition, the time required for 'mixing' of creatinine injected into the amniotic sac was longer with a dead than with a living fetus but its half-life averaged 4.5 h regardless of fetal status.