Third Trimester Umbilical Artery Doppler in Low-Risk Pregnancies and its Correlation to Estimated Fetal Weight and Birthweight. 
This study investigated the correlation between the umbilical artery (UA) pulsatility index (PI) and the estimated fetal weight percentile and birthweight (BW) percentile, respectively.  We included low-risk pregnancies, in which UA Doppler investigations after 28 weeks were performed.  Cases were allocated according to BW percentiles: small for gestational age (SGA) with BW < 10th percentile; appropriate for gestational age (AGA) and large for gestational age (LGA) with BW > 90th percentile.  We analyzed differences in the mean UA-PI and UA-PI z-score for gestational age according to the three groups.  Linear regression was performed to evaluate any relationship between Doppler indices and BW percentiles.  Multiple logistic regression analysis was performed to determine the independent association of UA-PI with LGA babies.  In a second step, we considered data on estimated fetal weight (EFW) percentiles and performed the same analysis.  We analyzed 14 554 pregnancies from 2004 to 2015.  The mean UA-PI and mean UA-PI z-scores in the LGA group were lower than in the AGA and SGA groups (p < 0.001).  UA-PI and UA-PI z-scores were linearly related to birthweight percentiles (p < 0.001) and to EFW percentiles (p < 0.001).  Logistic regression analysis showed that low UA-PI was independently associated with neonatal LGA (p < 0.001).  The higher the EFW and BW percentiles, the lower the UA-PI.  However, reference ranges for UA Doppler are only based on gestation weeks.  Further studies are needed to clarify whether customized reference ranges based on EFW percentiles are more appropriate for the evaluation of fetal wellbeing in the third trimester.