A group of patients with pregnancies complicated by pregnancy-induced hypertension (PIH) were treated with anti-hypertensive drugs labetalol and nifedipine. These subjects had umbilical artery Doppler flow velocity and resistance index measurements at 27-32 weeks gestation and again at 33-36 weeks gestation. An increase in resistance index measurements was noted over time. The authors attributed this increase in umbilical artery resistance to progression in placental impairment due disease progression rather than the medication management of PIH [13]. Another group of patients with pregnancies complicated by preeclampsia had significantly elevated umbilical artery systolic flow to diastolic flow (S/D) ratios compared to a control group. When treated with an antihypertensive agent alpha-methyldopa, there was a significant reduction in umbilical artery S/D ratios in the group of women with preeclampsia [14]. An in-vitro study was undertaken with isolated segments of human umbilical cords that were evaluated for measured umbilical artery resistance. The segments were exposed to varying concentrations of common substances that affect arterial tone and are used as antihypertensive agents such as labetalol, alpha-methyldopa, nifedipine, and magnesium sulfate. All except alpha-methyldopa demonstrated a dose-dependant reduction in umbilical artery resistance due to direct umbilical artery smooth muscle relaxation [15].