Pethidine blood concentrations at time of birth

Pethidine levels in maternal and fetal blood were estimated at birth after variable periods of intramuscular administration--delivery intervals in forty pregnant women. The results suggest that maternal--fetal equilibrium occurs at a fast rate with a regression coefficient initially of 0.54. There appeared to be a tendency towards trapping of the drug in the fetal circulation resulting in a rise in the regression coefficient after 3 h reaching 0.85. The pharmacokinetics and the clinical significance of these observations are discussed.