Placental transfer and disposition of bilirubin in the pregnant monkey

The placental transfer and disposition of conjugated and unconjugated bilirubin was studied in the pregnant rhesus monkey at term. The data indicate bidirectional placental transfer of unconjugated bilirubin, whereas only small quantities of conjugated bilirubin cross the placenta. The small quantity of unconjugated bilirubin excreted into the fetal biliary tract indicates that normally the placenta is the primary route of disposal of unconjugated bilirubin of fetal origin. Unconjugated bilirubin readily enters amniotic fluid from the maternal circulation. The data also indicate that the monkey fetus exhibits an impairment of hepatic excretion of conjugated bilirubin.