Routine assessment of cerebroplacental ratio at 35-37 weeks' gestation in the prediction of adverse perinatal outcome

Background: Third trimester studies in selected high-risk pregnancies have reported that low cerebroplacental ratio (CPR), due to high pulsatility index (PI) in the umbilical artery (UA), and or decreased PI in the fetal middle cerebral artery (MCA), is associated with increased risk of adverse perinatal outcomes.

Objective: To investigate the predictive performance of screening for adverse perinatal outcome by the cerebroplacental ratio (CPR) measured routinely at 35+6 - 36+6 weeks’ gestation.

Methods: This was a prospective observational study in 47,211 women with singleton pregnancies undergoing routine ultrasound examination at 35+6 - 36+6 weeks’ gestation, including measurement of UA-PI and MCA-PI. The measured UA-PI and MCA-PI and their ratio were converted to multiples of the median (MoM) after adjustment for gestational age. Multivariable logistic regression analysis was used to determine whether CPR improved the prediction of adverse perinatal outcome that was provided by maternal characteristics, medical history and obstetric factors. The following outcome measures were considered: first, adverse perinatal outcome consisting of stillbirth, neonatal death or hypoxic ischemic encephalopathy grades 2 and 3, second, presence of surrogate markers of perinatal hypoxia consisting of umbilical arterial or venous cord blood pH ≤7 and ≤7.1, respectively, 5-minute Apgar score <7, or admission to the neonatal intensive care unit for >24 hours, third, cesarean section for presumed fetal distress in labor, and fourth, neonatal birthweight <3rd percentile for gestational age.

Results: Low CPR was associated with increased risk of adverse perinatal outcome, presence of surrogate markers of perinatal hypoxia, cesarean section for presumed fetal distress in labor and birth of neonates with birthweight <3rd percentile. However, multivariable regression analysis demonstrated that the prediction of these adverse outcomes by maternal demographic characteristics and medical history was only marginally improved by the addition of CPR. The performance of low CPR in the prediction of each adverse outcome was poor, with detection rates of 13–26% and false positive rate of about 10%. In appropriate for gestational age (AGA) neonates with birthweight ≥10th percentile the predictive accuracy of CPR was low with positive and negative likelihood ratios (LRs) ranging from 1.21 to 1.82, and 0.92 to 0.98, respectively; although the accuracy was better in small for gestational age (SGA) neonates this was also low with positive LRs of 1.31 to 2.26 and negative LRs of 0.69 to 0.92. Similar values were obtained in fetuses classified as SGA and AGA according to the estimated fetal weight. In the prediction of adverse outcomes within two weeks, rather than at any stage, after assessment the detection rate was higher but this was achieved at higher false positive rate and therefore similar positive and negative LRs.

Conclusion: In pregnancies undergoing routine antenatal assessment at 35+0 - 36+6 weeks’ gestation measurement of CPR provides poor prediction of adverse perinatal outcome in both SGA and AGA fetuses. Consequently, there is no justification in a shift of the focus of prenatal care from identification of pregnancies with low estimated fetal weight to that of pregnancies with low CPR.

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