Comparison of methods for identifying small-for-gestational-age infants at risk of perinatal mortality among obese mothers: a hospital-based cohort study

Objective: To assess differences in small-for-gestational age (SGA) classifications for the detection of neonates with increased perinatal mortality risk among obese women and subsequently assess the association between prepregnancy body mass index (BMI) status and SGA. Design: Hospital-based cohort. Setting: Twelve US clinical centres (2002–08). Population: A total of 114 626 singleton, nonanomalous pregnancies. Methods: Data were collected using electronic medical record abstraction. Relative risks (RR) with 95% CI were estimated. Main outcome measures: SGA trends (birthweight < 10th centile) classified using population-based (SGA_POP), intrauterine (SGA_IU) and customised (SGA_CUST) references were assessed. The SGA-associated perinatal mortality risk was estimated among obese women. Using the SGA method most associated with perinatal mortality, the association between prepregnancy BMI and SGA was estimated. Results: The overall perinatal mortality prevalence was 0.55% and this increased significantly with increasing BMI (P < 0.01). Among obese women, SGA_IU detected the highest proportion of perinatal mortality cases (2.49%). Perinatal mortality was 5.32 times (95% CI 3.72–7.60) more likely among SGA_IU neonates than non-SGA_IU neonates. This is in comparison with the 3.71-fold (2.49–5.53) and 4.81-fold (3.41–6.80) increased risk observed when SGA_POP and SGA_CUST were used, respectively. Compared with women of normal weight, overweight women (RR = 0.82, 95% CI 0.78–0.86) and obese women (RR = 0.80; 95% CI 0.75–0.83) had a lower risk for delivering an SGA_IU neonate. Conclusion: Among obese women, the intrauterine reference best identified neonates at risk of perinatal mortality. Based on SGA_IU, SGA is less common among obese women but these SGA babies are at a high risk of death and remain an important group for surveillance. Tweetable abstract: SGA is less common among obese women but these SGA babies are at a high risk of death.