The risk of late-onset fetal growth restriction and unfavorable neonatal outcomes among fetuses within the 10th-25th weight percentile: a prospective cohort study

BACKGROUND: While negative neonatal outcomes are more frequent in fetuses with estimated fetal weight (EFW) < 10%, they also occur often in fetuses with appropriate-for-gestational-age. This study aimed to evaluate the neonatal outcomes in 10th-25th percentile weight fetuses versus > 25th-<90th percentile. METHODS: This prospective cohort study took place at the Maternal-Fetal Medicine Department of a tertiary center in Tehran, Iran, from May 2024- March 2026. Low risk pregnant mothers aged 18-39 years, with singleton pregnancies, exhibiting EFW of 10th-90th percentile, were classified into: 10th-25th percentile group and > 25th-<90th percentile group. Both groups underwent biometric ultrasound at 32-33 weeks of gestation and additionally received biometric and fetal vascular Doppler ultrasound three weeks afterwards. We evaluated the two groups on: (1) Doppler parameters including umbilical artery pulsatility index (UA-PI), middle cerebral artery pulsatility index (MCA-PI), average uterine artery PI, and cerebroplacental ratio (CPR), (2) adverse perinatal and neonatal outcomes including fetal growth restriction (FGR), intrauterine fetal demise, oligohydramnios, meconium-stained liquor, low birth weight (LBW), cesarean section due to fetal distress, abnormal cord arterial blood gas (ABG), low APGAR score, neonatal intensive care unit (NICU) admission, respiratory distress syndrome (RDS), and neonatal mortality. We examined whether the Doppler indices had any predictive value for adverse outcomes in 10th-25th percentile weight fetuses. RESULTS: Of 402 individuals, 320 (79.6%) were in the > 25th-<90th percentile group, while 82 (20.4%) were in the 10th-25th percentile group. The incidence of negative perinatal and neonatal outcomes like FGR (25.60% vs. 2.81%), LBW (20.73% vs. 0.31%), RDS (13.41% vs. 5.31%), low APGAR scores (14.63% vs. 5.31%), abnormal ABG (13.41% vs. 3.75%), and NICU admission (28.04% vs. 9.37%) was higher in the 10th-25th percentile group compared to the > 25th-<90th percentile group. Although abnormal UA-PI, abnormal MCA-PI, and abnormal CPR were more frequently found in fetuses weighing in the 10th-25th percentile group; however, fetal Doppler indices did not successfully predict negative outcomes for fetuses in this weight range. CONCLUSIONS: Adverse perinatal and neonatal outcomes were higher in fetuses within 10th-25th percentile for birth weight, with abnormal Doppler indices more common, but these indices did not predict negative outcomes effectively.