Fetal and Neonatal Outcomes in Fetuses with an Estimated Fetal Weight Percentile of 10–20 in the Early Third Trimester: A Retrospective Cohort Study

<b>Background:</b> Fetal size is often dichotomized as normal or abnormal using the 10th percentile of estimated fetal weight (EFW) or abdominal circumference as a cutoff. While the risk of adverse perinatal outcomes decreases with increasing fetal weight percentile, no percentile completely eliminates that risk. <b>Objective:</b> The aim of this study was to compare perinatal outcomes between fetuses with an EFW between the 10th and 20th percentiles and those with an EFW between the 20th and 90th percentiles (i.e., >20 and <90) at the beginning of the accelerated growth stage (28.0-30.0 weeks' gestation). <b>Methods:</b> We conducted a retrospective cohort study of singleton pregnancies managed at a quaternary center in Mexico City (2017-2024). Outcomes were compared based on EFW percentiles at 28.0-30.0 weeks. The primary outcome was adverse neonatal outcome (ANeO), defined as the presence of at least one of the following: umbilical artery pH ≤ 7.1, 5 min Apgar ≤ 7, NICU admission, early neonatal hypoglycemia, non-reassuring fetal status, respiratory distress syndrome, intraventricular hemorrhage, hypoxic-ischemic encephalopathy, or perinatal death. Secondary outcomes included progression to fetal growth restriction (FGR) and low birth weight. Modified Poisson regression was used to estimate adjusted risk ratios (aRRs) with 95% confidence intervals (CIs). <b>Results:</b> Among 650 cases, ANeO occurred in 45.8% of fetuses in the 10th-20th percentile group vs. 29.4% in the 20th-90th percentile group (aRR: 1.51, 95% CI: 1.22-1.86; <i>p</i> < 0.001). FGR and low birth weight were also more frequent in the 10th-20th percentile group (21.1% and 27.6% vs. 6.4% and 5.8%, respectively; <i>p</i> < 0.001). <b>Conclusions:</b> Fetuses between the 10th and 20th percentiles at 28-30 weeks have increased risks of neonatal morbidity, FGR, and low birth weight.