Color Doppler imaging of the uteroplacental circulation in the middle trimester: observations on the development of a low‐resistance circulation

We aimed to study the changes in the flow dynamics of the uteroplacental and umbilical circulations in the middle trimester of normal pregnancy, and establish normal ranges for indices of the Doppler flow velocity waveforms (FVWs) from both circulations at 14, 18 and 24 weeks. A longitudinal study was conducted with the use of color Doppler imaging to localize both uterine arteries and umbilical arteries and obtain FVWs from 106 healthy nulliparous women with a singleton pregnancy. Of these, 70 did not have a complicated pregnancy outcome, defined as hypertension, growth retardation, preterm delivery or perinatal death. We measured the resistance index (RI), pulsatility index (PI), systolic/diastolic (S/D) ratio and presence of early diastolic notching in both uterine arteries, and RI, PI and S/D in one umbilical artery. The results showed a fall in all measured indices of blood flow impedance in the uteroplacental and umbilical circulations and a marked reduction in the incidence of the early diastolic notch in the uterine artery FVWs during the mid‐trimester. Our conclusion was that color Doppler imaging allows for the accurate localization of both uterine and umbilical arteries. Doppler FVWs then obtained confirm the development of the low‐resistance uteroplacental and umbilical circulations in the mid‐trimester. Diastolic notching is common at 14 weeks in normal pregnancy but uncommon at 24 weeks.