Relationship between prolonged gestation and nifedipine pharmacokinetics in long‐term tocolysis

In this study, we examined the pharmacokinetics of nifedipine and investigated the maternal and foetal background factors that prolong pregnancy in pregnant women undergoing long‐term tocolysis. This prospective observational study included 38 pregnant women hospitalised for threatened preterm labour and treated with nifedipine extended‐release tablets in combination with an intravenous ritodrine infusion. Maternal plasma nifedipine concentrations were determined using high‐performance liquid chromatography. All patients were administered 20 or 40 mg/dose of nifedipine every 6 h at the time of blood sampling. The plasma trough concentration (Ctrough) was 22.6 ± 17.3 ng/mL, the maximum plasma concentration (Cmax) was 30.9 ± 15.3 ng/mL and the time to maximum concentration (Tmax) was 1.70 ± 1.10 h, as determined using noncompartmental analysis (NCA). The area under the curve for drug concentration (AUCtau) was 152.3 ± 91.8 mg/L・h, and oral clearance (CL/F) was 0.17 ± 0.08 L/h. Using logistic regression analyses, we identified the factors that predicted term delivery from 37 weeks to