BACKGROUND/OBJECTIVES: Successful pregnancy requires the de novo creation of low-resistance utero-placental and feto-placental circulations and incomplete remodeling of this vasculature can lead to maternal or fetal compromise. Maternal BMI and fetal sex are known to influence vascular compliance and placental development, but it is unknown if these are independent or synergistic effects. Here we aim to investigate the impact of maternal obesity, fetal sex, and any interaction thereof on maternal cardiovascular adaptation to pregnancy, by assessing the physiological drop of uterine artery doppler pulsatility (UtA-PI) and umbilical artery doppler pulsatility index (UA-PI) over gestation. SUBJECTS/METHODS: Nulliparous women with a singleton pregnancy participating in a prospective cohort study (n = 4212) underwent serial UtA-PI and UA-PI measurements at 20-, 28- and 36-weeks gestation. Linear mixed regression models were employed to investigate the influence of maternal BMI, fetal sex and interactions thereof on the magnitude of change in UtA-PI and UA-PI. RESULTS: Throughout gestation, UtA-PI was higher for male fetuses and UA-PI was higher for female fetuses. The physiological drop of UtA-PI was significantly smaller in overweight (change -24.3% [95%CI -22.3, -26.2]) and obese women (change -21.3% [-18.3, -24.3]), compared to normal-weight women (change -25.7% [-24.3, -27.0]) but did not differ by fetal sex. The physiological drop in UA-PI was greater for female than male fetuses (-32.5% [-31.5, -33.5] vs. -30.7% [-29.8, -31.7]) but did not differ by maternal BMI. No interactions between maternal BMI and fetal sex were found. CONCLUSIONS: Maternal cardiovascular adaptation to pregnancy is independently associated with maternal BMI and fetal sex. Our results imply sexual dimorphism in both maternal cardiovascular adaptation and feto-placental resistance.