Serum levels of ritodrine used for the management of spontaneous preterm labour.

Ritodrine 0·05 mg/min gave a mean scrum level of 3·6ng/ml and 0·55mg/min gave a mean level of 204 ng/ml, when infused intravenously into women in spontaneous preterm labour. Oral administration of ritodrine, 5 mg 3 hourly, gave a mean serum level of 0·3ng/ml; 10 mg 2 hourly gave 1·3ng/ml. The drug crosses the placenta and umbilical vein cord serum levels were higher than maternal levels when it was continued until delivery. Ritodrine was detected in such a baby 24 hours after birth.