The emergence of the novel H1N1 influenza virus has raised concern among public health officials and the public alike. Although initial reports suggested that the risk of serious disease or death was less than initially feared, continued experience suggests that the disease will have a major impact on the public's health. To address this urgent public health challenge, the centers of the Obstetric-Fetal Pharmacology Research Units Network (OPRU) will recruit a total of 75 pregnant women (25 per trimester with singletons pregnancies) who are receiving oseltamivir for supposed influenza infection. When possible, we will recruit women with twin gestation to determine if fetal number affects the drugs pharmacokinetics. We do not anticipated a significant number of multi-fetal gestations (< 10) but even limited data might prove useful in this population given the tremendous change in renal drug clearance that is normally seen in this group of women. An additional 25 healthy non-pregnant women will be studied to define pregnancy- related changes in the PK of oseltamivir and the carboxylate.