Pharmacokinetics of betamethasone after maternal or fetal intramuscular administration

Objective The purpose of this study was to determine the pharmacokinetics of betamethasone in maternal and fetal circulations after maternal or fetal intramuscular administration. Study design Ewes that bore single fetuses underwent surgery at approximately 96 days of pregnancy for the implantation of fetal and maternal vascular catheters. At approximately 103 days, five ewes were injected intramuscularly with betamethasone (0.5 mg/kg body weight) or five fetuses received ultrasound-guided intramuscular injections of betamethasone (0.5 mg/kg estimated fetal weight). Maternal and fetal blood samples were collected serially for the measurement of plasma betamethasone concentrations. Results Fetal injection caused higher peak fetal betamethasone concentrations (341.2±23.7 nmol/L) than maternal injection (37.6±3.7 nmol/L; P<.001) and greater cumulative betamethasone exposure. The half-life of betamethasone in the fetal circulation was shorter after fetal injection (1.1±0.3 hours) than after maternal injection (8.5±2.0 hours; P = .006). Conclusion The duration of fetal and maternal exposure to betamethasone can be minimized by direct fetal intramuscular administration that, in sheep, affords lung maturation without adverse effects on fetal growth.