Background
Trophoblastic invasion of the uterine spiral arteries substantially increases compliance to accommodate increased blood flow to the placenta. Failure of this process impedes uterine artery blood flow, and this may be detected by uterine artery Doppler flow studies. However, the clinical utility of uterine artery Doppler flow studies in the prediction of adverse pregnancy outcomes in a general population remains largely unknown.
Objective
To determine the utility of early second trimester uterine artery Doppler studies as a predictor of small for gestational age (SGA) neonates.
Study Design
Nulliparous women with a viable singleton pregnancy were recruited during their first trimester into an observational prospective cohort study at eight institutions across the United States. Participants were seen at three study visits during pregnancy and again at delivery. Three indices of uterine artery Doppler flow (resistance index, pulsatility index, and diastolic notching) were measured in the right and left uterine arteries between 16 weeks 0 days and 22 weeks 6 days gestation. Test characteristics for varying thresholds in the prediction of SGA (defined as birth weight <5th percentile for gestational age [Alexander growth curve]) were evaluated.
Results
Uterine artery Doppler indices, birth weight, and gestational age at birth were available for 8,024 women. Birth weight <5th percentile for gestational age occurred in 358 (4.5 percent) of the births. Typical thresholds for the uterine artery Doppler indices were all associated with birth weight <5th percentile for gestational age (P<0.0001 for each), but the positive predictive values for these cutoffs were all <15 percent and areas under receiver operating characteristic curves (AUCs) ranged from 0.50 to 0.60. Across the continuous scales for these measures, the AUCs ranged from 0.56 to 0.62. Incorporating maternal age, early pregnancy BMI, race/ethnicity, smoking status prior to pregnancy, chronic hypertension, and pre-gestational diabetes in the prediction model resulted in only modest improvements in the AUCs ranging from 0.63 to 0.66.
Conclusion
In this large prospective cohort, early second trimester uterine artery Doppler studies were not a clinically useful test for predicting SGA babies.