The utility of an obesity-specific small for gestational age standard for pregnancies complicated by maternal obesity^*

Introduction: An obesity-specific standard for small for gestational age (SGA) pregnancies may help identify additional at risk pregnancies. Methods: This was a retrospective cohort study of all non-anomalous singleton neonates born in Texas from 2006–2011. Analysis was limited to births between 34 and 42 weeks gestation. Two SGA birth weight standards (birth weight ≤10th centile) were generated, one using the entire population (SGA_pop) and another using obese pregnancies (SGA_cust). The outcomes of interest included: risks of stillbirth, neonatal death, 5-minute Apgar score below 7, NICU admission, and assisted ventilation >6 h. Results: Using the population standard, the prevalence of SGA complicated by obesity was 8.1%, compared with 10.3% using the obesity-specific standard. 10,457 additional pregnancies were identified as SGA. Compared to obese AGA pregnancies, the aHR for stillbirth was 5.45 [4.28, 6.94] for SGA_pop, and 1.21 [0.54, 2.74] for SGA_cust-pop. The risks for the following neonatal complications were slightly higher for SGA_cust-pop group compared to AGA group: neonatal death aOR 1.40 [1.05, 1.87], low 5-minute Apgar 1.31 [1.09, 1.57], and NICU admission 1.13 [1.03, 1.25]. These risks were lower than SGA_pop. Conclusions: Using an obesity-specific SGA standard, a subgroup of pregnancies with marginally increased risk for neonatal complications was identified.