Chemical dilution and clearance studies to estimate amniotic fluid volume and amniotic fluid ingestion in normal...

Amniotic fluid volume (AFV) and amniotic fluid ingestion rate (or fetal swallowing rate, FSR) were estimated by inulin and para-aminohippurate (PAH) dilution in 14 normal baboon pregnancies. Mean (± SE) AFV was significantly lower at 137-140 days of pregnancy (preterm) than at 173-178 days (term) (inulin: 326 ± 22.9 ml vs 483 ± 55.9 ml, P = 0.014; PHA:269 ± 39.4 ml vs 471 ± 39.4 ml, P = 0.002). In proportion to fetal weight, however, mean AFV was similar throughout the third trimester of pregnancy (inulin: 582 ± 40.9 ml/kg; PAH: 541 ± 39.8 ml/kg). Mean FSR was lower in preterm than in term animals when estimated by inulin dilution (587 ± 55.5 ml/day vs 784 ± 55.0 ml/day, P = 0.030) but not when estimated by PAH dilution (753 ± 65.7 ml/day vs 625 ± 50.6 ml/day). In proportion to their weights, however, preterm fetuses swallowed amniotic fluid more rapidly than term fetuses (inulin: 1,216 ± 117.6 ml/kg/day vs 840 ± 67.5 ml/kg/day, P = 0.025; PAH: 1,561 ± 142.9 ml/kg/day vs 682 ± 62.7 ml/kg/day, P < 0.001). Furthermore, our data suggest that the commonly accepted technique for estimating AFV may be based on inaccurate premises, that insulin may be a better marker than PAH to estimate AFV and FSR, and that needle aspiration of amniotic fluid does not appear to be an adequate technique to validate chemical dilution methods. Our data, however, provide estimates which indicate that the baboon is an appropriate animal model in which to seek refinements and validation of our techniques.