Umbilical cerebral ratio for detection of poor neonatal acid–base status in small-for-gestational age late preterm fetuses

Objectives To assess the umbilical-cerebral ratio (UC) in the prediction of abnormal acid–base status at birth in a cohort of small for gestational age (SGA) late preterm fetuses. Methods Retrospective cohort study. From 2011 to 2015 we included singleton fetuses with estimated birthweight or fetal abdominal circumference ≤ 10th centile, delivered between 32+1 and 37+0 weeks' gestation. Pregnancies complicated by fetal abnormalities, aneuploidy or stillbirth were excluded. The umbilical artery pulsatility index (UA-PI), middle cerebral artery pulsatility index (MCA-PI), and umbilical arterial cord blood pH (UA-pH) were recorded. UC was calculated by dividing UA-PI by MCA-PI. Values >1 were considered abnormal. Poor neonatal acid–base status at birth was defined as UA-pH < 7.1. To limit the treatment paradox we analysed only cases where fetal Doppler were assessed at the time of presentation, between 30 and 8 days from delivery. The birthweight (BW) values and Doppler parameters were converted into Z scores adjusting for gestational age using reference ranges. Results The study cohort included 95 fetuses. There were 4/95 cases with UA-pH < 7.1. Median gestational age at delivery was 35+1 weeks (IQR 34+0 to 36+2), median BW Z score was −1.5 (IQR −2.08 to −0.79). The UC ratio at the time of presentation was >1 in 7/95 cases. The relative risk of abnormal acid–base status at birth with abnormal UC was 12.6 (p = 0.02). The sensitivity and specificity of high UC for predicting UA-pH <7.1 were 33.3% and 96.3%; likelihood ratio (+) 7.7, likelihood ratio (−) 0.7. Post-test probability of UA-pH <7.1 with abnormal UC increased from 4% to 24%. Conclusions Although abnormal UC increased moderately the probability of UA-pH <7.1 at birth; this test performs poorly for the prediction of abnormal acid–base status in SGA late preterm newborns.