ObjectiveTo assess the nitric oxide synthase activity in placentas from women with either normal or abnormal Doppler ultrasound umbilical artery flow velocity waveforms who delivered by elective cesarean.MethodsThis prospective observational study involved 16 women admitted either for elective cesarean for standard obstetric indications (with normal umbilical artery Doppler waveform studies,n= 8) or with evidence of fetal or maternal complications of pregnancy (with abnormal umbilical artery Doppler studies,n= 8). Placental tissue was collected and frozen in liquid nitrogen immediately upon delivery. Following storage at −80C, nitric oxide synthase activity was analyzed by measuring the conversion of [3H]L-arginine to [3H]L-citrulline.ResultsPlacentas from women with abnormal umbilical artery flow velocity waveforms showed significantly lower mean nitric oxide synthase activity than did placentas from women with normal umbilical artery flow velocity waveforms (Vmax= 11.3 pmol/minute/mg protein versus 22.6 pmol/minute/mg protein).ConclusionThere is a statistically significant reduction in nitric oxide synthase activity in placentas from pregnancies with abnormal umbilical artery flow velocity waveforms.