Effects of aspirin and acetaminophen in pregnancy and in the newborn.

Inhibitors of prostaglandin synthesis are increasingly recommended for delaying premature delivery. However, such inhibitors--eg, aspirin, indomethacin, or naproxen--may interfere with uterine contractility, with maternal, fetal, and neonatal platelet function, and especially with fetal circulation and postnatal adaptation. The instillation of aspirin (50 to 90 mg/kg of fetal body weight) into the rumina of fetal lambs resulted in significant increase of pulmonary arterial pressure, which was directly related to constriction of the ductus arteriosus. Such pressure elevation may act as a stimulus to increased muscular development in the small vessels of the lung, which, in turn, could interfere with the rapid reduction in pulmonary vascular resistance normally occurring after birth. The effects of acetaminophen in pregnancy and in the newborn have not been studied extensively; when ingested in usual therapeutic doses, the available data have shown no adverse effects.