Bilirubin Metabolism in the Fetus*

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Abstract
A technique is described for delivery of viable guinea pig fetuses with intact placental circulation. This experimental model was used to determine the permeability of the placenta for unconjugated and conjugated bilirubin-Cl4 injected into the fetal circulation. Unconjugated bilirubin-C14 rapidly disappeared from the fetal plasma, and 52 to 66% of the administered radioactivity was excreted in the maternal bile within 2 hours. By contrast, disappearance of conjugated bilirubin-C14 was slower, and less than 4% of the injected pigment dose appeared in the maternal bile. On simultaneous administration of unconjugated bilirubin-C14 and albumin-I131 to the fetus, only the pigment was transferred across the placenta, whereas virtually no labeled albumin was detectable in the maternal circulation. In the fetus, biliary excretion of unconjugated and conjugated bilirubin-C14 was impaired to a similar degree, suggesting that, in addition to deficient conjugation, the fetal liver also exhibits an excretory defect.