Local cerebroplacental ratio reference ranges are better predictors for adverse delivery outcomes in normal weight fetuses during pregnancy

Objective: To evaluate the predictive value of local versus external cerebroplacental ratio (CPR) reference ranges for delivery outcomes in low-risk pregnancies.

Methods: A retrospective analysis of all feto-maternal demographic and biometric data in fetuses with normal estimated fetal weight (EFW) and a CPR examination between the years 2014-2019, in a university medical center. The study group included healthy singleton pregnancies from 32-week gestation, with an examination-to-delivery interval of <31 days. The three models compared two thresholds: <5th percentile (CPR 1, CPR 3) and <10th percentile (CPR2). The CPR1 and CPR2 models both use local CPR reference ranges, while the CPR3 model uses an external CPR reference range. The main outcome was predictive accuracy for urgent cesarean delivery (CD), operative delivery (OD), and composite outcome (CO), defined as an Apgar score of <7, fetal blood pH < 7.1 or admission to the neonatal intensive care unit (NICU).

Results: Overall, 410 low-risk pregnancies with normal weight fetuses were enrolled in the study. All three CPR models turned out to be significant predictors of CD, with an odds ratio (OR) of 9, 95% CI (2.7-27), p < .001 for CPR1, and an OR of 2.9, 95% CI (1.1-7.4), p < .04 for CPR2, and an OR of 3.4, 95% CI (1.7-6.8), p < .001 for CPR3. All the three models were also found to be predictors of OD, and an OR of 6.9, 95% CI (2.1-22) p < .04 for CPR1, and an OR of 2.8, 95% CI (1.2-6.7), p < .04 for CPR2, and an OR of 2.8, 95% CI (1.4-5.3) p < .01 for CPR3. The positive predictive values (PPV) for CD and OD were both 50% for CPR1, versus 28% and 26% in CPR2, and 24% and 25% in CPR3. The negative predictive value (NPV) was similar, around 88% in all three models. None of the models were found to be significant predictors for CO.

Conclusions: A CPR model based on local reference ranges and <5th percentile cutoffs showed the highest PPV for CD and OD. The calculation of local references for CPR should be encouraged.