Meperidine disposition in mother, neonate, and nonpregnant females.

Whether meperidine metabolism is affected by pregnancy or immaturity has not been clearly established. This is of interest because meperidine is commonly given during labor for pain relief and the fetus receives the drug in utero. Moreover, animals studies suggest that the hormones of pregnancy contribute to decreased activity of the drug-metabolizing enzymes. In our study gas chromatography was used to determine the concentrations of meperidine and normeperidine in the plasma and urine of pregnant and nonpregnant women and in the urine of neonates. Plasma samples were collected for at least 3 hr after a dose of meperidine intravenously to calculate the kinetic parameters of meperidine disposition; urine samples were collected for 3 days. In contrasts to reports on animals, we found that pregnant and nonpregnant women readily metabolize meperidine to normeperidine and excrete both in a similar manner. No significant differences were demonstrated between any of the kinetic constants for peripartum and nonpregnant subjects. The neonate was found to metabolize and excrete these drugs less rapidly.