Placental transfer and fetal urinary excretion of gentamicin during constant rate maternal infusion.

Placental transfer and fetal urinary excretion of gentamicin was studied in midtrimester goat and human previable fetuses during constant rate maternal infusion with the drug. Gentamicin was not detected in the serum of any of the goat fetuses, even when maternal serum concentrations ranged from 15.2 mug/ml to 20.9 mug/ml. However, gentamicin was present in the amniotic fluid of four animals. Gentamicin was also present in fetal urine collected from three animals. In contrast human fetal central venous serum concentrations of gentamicin were 21-37% of those in maternal serum after constant rate infusion of the mother. In addition, gentamicin was present in human fetal urine in concentrations 2-- 3 times those in fetal serum. The observed difference in fetal serum concentration of gentamicin between the two species represents a difference in placental permeability to gentamicin and/ or a difference in fetal renal clearance of the drug.