Small-for-gestational-age infants among uncomplicated pregnancies at term: a secondary analysis of 9 Maternal-Fetal Medicine Units Network studies

Background: Most small (birthweight <10%) for-gestational-age cases occur at term, in uncomplicated pregnancies, and are not identified during prenatal visits as having fetal growth restriction. Hence, they do not benefit from antepartum surveillance and timed delivery. There is dismissive and disquieting opinion that small for gestational age among uncomplicated pregnancies is not associated with increased morbidities and, therefore, does not warrant improved detection. Our hypothesis was that among uncomplicated pregnancies at term, small for gestational age have significantly higher morbidity and mortality than appropriate (birthweight 10-89%) for gestational age.

Objective: We sought to compare composite neonatal morbidity among uncomplicated term singleton pregnancies with small vs appropriate for gestational age.

Study design: We culled collected data from 9 completed Maternal-Fetal Medicine Units studies conducted from 1989 through 2004. All data were collected prospectively by trained staff. We excluded women who delivered <37 weeks and those with hypertension or diabetes, multiple gestation, known anomalies, and birthweight of ≥90% for gestational age. Using multivariable analysis, we compared composite neonatal morbidity, which included stillbirth and neonatal mortality between small and appropriate for gestational age. Random effect logistic regressions were used to account for study heterogeneity, with adjustment for potential confounders. We calculated adjusted odds ratios and 95% confidence intervals.

Results: Of the >91,000 women enrolled in the studies, 60% (n = 50,011) met the inclusion criteria. Among the uncomplicated pregnancies, 10.8% (n = 5416) were small for gestational age. The rate of composite neonatal morbidity of 16% in small for gestational age and 10% in appropriate for gestational age persisted (adjusted odds ratio, 1.75; 95% confidence interval, 1.71-1.78). After adjustment for confounders, the following neonatal morbidities were significantly more common among term small than appropriate for gestational age: Apgar <4 at 5 minutes, respiratory distress syndrome, mechanical ventilation, necrotizing enterocolitis grade 2 or 3, and neonatal sepsis. Lastly, rate of stillbirths (3.5 vs 0.9/1000 births; adjusted odds ratio, 3.49; 95% confidence interval, 1.83-6.67) and neonatal mortality (1.1 vs 0.4/1000 births; adjusted odds ratio, 2.56; 95% confidence interval, 1.83-3.57) were significantly more common with small than appropriate for gestational age. In secondary analyses the composite neonatal morbidity among newborns at <5% and at 5-9% was significantly higher than appropriate for gestational age. Lastly, in subgroup analyses of women who delivered at 37.0-38.6 weeks or at ≥39.0 weeks, the increased rate of composite neonatal morbidity, stillbirth, and neonatal mortality among small for gestational age persisted.

Conclusion: Among uncomplicated pregnancies at term, small- compared to appropriate-for-gestational-age newborns have a significantly higher likelihood of composite neonatal morbidity, stillbirth, and neonatal mortality. A large multicenter trial is warranted to determine if improved detection of small for gestational age among uncomplicated pregnancies can mitigate morbidities and mortality, without disproportionate interventions and iatrogenic complications.