Clinical utility of conditional growth centiles and fetal body proportions in predicting neonatal anthropometry and perinatal outcomes: A prospective observational study from a tertiary referral centre

Background Conventional size-based growth charts have limited ability to identify fetuses at risk of adverse perinatal outcomes (APNO), highlighting the need for improved antenatal growth assessment strategies. Objective To evaluate whether fetal conditional centiles and fetal body-proportions outperform traditional size centiles in predicting abnormal neonatal anthropometry and APNO in a mixed-risk obstetric cohort. Methods A prospective observational study of 500 singleton pregnancies was conducted at a tertiary care centre in South India. Conditional centiles were calculated using estimated fetal weight at 17-22 weeks as reference, and head circumference/abdominal circumference (HC/AC) ratios were assessed. Two population-based growth charts were used for comparison: the multiethnic Intergrowth-21st standard (Chart 1) and a locally derived South Indian Sonocare-based growth chart (Chart 2). Results Among 500 enrolled pregnancies, 42% of fetuses were <10th conditional centile and 42% had high HC/AC-ratios. All antenatal predictors were associated with small for gestational age (SGA) neonates. Population-based size charts showed strongest associations (Chart-1:aOR≈10) with low ponderal index. Conditional centiles showed high sensitivity (87%) but low-PPV (26.21%) for identifying SGA. Only Chart-1 SGA classification and high HC/AC ratio were significantly associated with composite APNO. An HC/AC ratio >1.01 was also independently associated with neonatal SGA, reduced birth length (BL), and reduced HC. However, none of the antenatal predictors demonstrated significant associations with severe APNO. ROC analysis showed that HC/AC ratio had only modest discriminatory ability for neonatal SGA (AUC: 0.68; 95% CI: 0.61-0.75 ), reduced BL (AUC: 0.59; 95% CI: 0.53-0.65), and composite APNO (AUC: 0.57; 95% CI: 0.50-0.64). Conclusion Conditional centiles and HC/AC ratios improved sensitivity for detecting SGA but did not predict APNO. Population-based size centiles maintained stronger associations with neonatal SGA and APNO, although overall sensitivity remained limited. These findings highlight that fetal size, growth trajectory, and body proportions explain only part of the multifactorial risk underlying APNO, underscoring the need for integrated approaches combining biometry with Doppler and placental biomarkers to enhance risk stratification.