The high-risk minority: undetected preterm SGA disproportionately contributes to adverse outcomes in a Chinese cohort

Small for gestational age (SGA) infants face higher risk of adverse outcomes compared to appropriate for gestational age (AGA) infants. Whether undetected SGA leads to worse clinical outcomes than detected SGA remains debated. This study compared perinatal outcomes between undetected and detected SGA in preterm and term pregnancies, and identified risk factors for missed diagnoses.

Singleton deliveries > 24 weeks at Peking University Third Hospital (January 2018-June 2023) were included. In this single-center retrospective study, participants were stratified into three groups: detected SGA, undetected SGA and AGA (matched by gestational age and delivery date with undetected SGA). Analysis was conducted in preterm and term deliveries respectively. Multivariate logistic regression models were conducted to compared neonatal outcomes between detected SGA and undetected SGA. Furthermore, differences in maternal demographic and clinical characteristics between AGA and undetected SGA were analyzed.

Among 32158 deliveries, 1509 SGA infants were identified, including 679 detected prenatally and 830 undetected. Adverse outcomes occurred in 112 and 60 cases in these groups, respectively. Notably, preterm undetected SGA, representing only 6.7% (56/830) of undetected cases, accounted for 65% (39/60) of all adverse outcomes in that group. Compared to detected SGA, undetected SGA had significantly higher rates of composite adverse outcomes in both preterm (aOR 3.869, 95% CI: 1.919-7.801) and term deliveries (aOR 3.308, 95% CI: 1.236-8.855). In preterm infants, undetected SGA specifically increased risks of neonatal cerebral damage, neonatal respiratory distress syndrome, and necrotizing enterocolitis. Analysis of risk factors for undetected SGA (vs. AGA n = 830) revealed that among preterm deliveries, higher maternal BMI (aOR 1.135, 95% CI: 1.016-1.267), hypertensive disorders (aOR 4.473, 95% CI: 1.872-10.688), and gestational diabetes mellitus (GDM) (aOR 8.371, 95% CI: 1.544-45.397) were associated with increased risk. Among term deliveries, hypertensive disorders, GDM, amniotic fluid index < 8 cm, uterine abnormality, nuchal cord, and placenta previa/accrete were significant risk factors.

Undetected SGA was associated with adverse neonatal outcomes. Preterm undetected SGA carried a disproportionately high morbidity burden compared to term undetected SGA. Prenatal factors, including maternal complications, umbilical cord and placental abnormalities, played a crucial role in identifying SGA.