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
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 (SGAPOP), intrauterine (SGAIU) and customised (SGACUST) 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, SGAIU 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 SGAIU neonates than non-SGAIU 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 SGAPOP and SGACUST 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 SGAIU neonate.Conclusion
Among obese women, the intrauterine reference best identified neonates at risk of perinatal mortality. Based on SGAIU, 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.