Changes in uterine blood flow during human pregnancy.

A transvaginal duplex Doppler ultrasonography system was used to measure blood flow characteristics in the ascending uterine artery before and during pregnancy. The system uses a 5 MHz Doppler transducer coupled to a 6.5 MHz imaging probe. There was a steady increase in volume flow rate in the left ascending uterine artery from a mean of 94.5 ml/min before pregnancy to a mean of 342 ml/min in late gestation (reflecting a 3.5-fold increase). With the assumption of equal flow on both uterine arteries, the fraction of the cardiac output that is distributed to these vessels was calculated to be 3.5% in early pregnancy and to reach 12% near term. The mean diameter of this vessel in the nonpregnant state was 1.6 mm, increasing to 3.7 mm toward term. The resistance to flow, expressed as the peak systolic to end-diastolic flow velocity ratio, declined from a mean of 5.3 in the nonpregnant state to a mean of 2.3 near term. These changes reflect the perpetual growth and development of the uteroplacental circulation, which provides the metabolic demands of the growing fetus throughout gestation.