Persistent pulmonary hypertension of the newborn (PPHN), previously called persistent fetal circulation, is a birth defect affecting approximately 1 in 1250 liveborn term infants; even with complex and high-risk interventions, PPHN results in substantial mortality and morbidity. This defect results from the inappropriate muscularization of fetal pulmonary vessels, and experimental and human evidence consistently suggests that maternal cigarette smoking and antenatal exposure to NSAIDs, particularly aspirin or ibuprofen, may play a role in the etiology of this condition. Because these exposures are quite prevalent (e.g., ibuprofen is currently taken in the first trimester or later in pregnancy by 15 percent and 3.2 percent of women, respectively), testing these hypotheses is of considerable public health importance.