Population standards of birth weight underestimate fetal growth abnormalities in diabetic pregnancies

The objective of this study was to compare the frequency of abnormal fetal growth in women with diabetes mellitus (DM) using population-based birth weight (pop BW) percentiles compared with customized birth weight (cust BW) percentiles, which include adjustments for maternal race, parity, height, weight, and fetal sex. The study design comprised a retrospective cohort of singleton DM pregnancies delivered over a 1-year period (June 2007 to May 2008) from a single tertiary care university-based medical center. Inclusion criteria were gestational age >20 weeks at delivery, live birth, and absence of major chromosomal/structural abnormalities. Small for gestational age (SGA), <10th percentile, and large for gestational age (LGA), >90th percentile pregnancies were categorized based on pop BW or cust BW standards. There were significant differences in the rates of SGA (p<0.004) and LGA (p<0.001) between cust BW and pop BW methods. When comparing the two methods, pop BW did not identify 13/16 (81%) of SGA and 23/39 (59%) of LGA babies defined by cust BW methods. The use of cust BW calculation in a diabetic population identified a greater percentage of neonates with pathologic fetal growth compared with pop BW standards, suggesting that the population standard may underdiagnose abnormal fetal growth in diabetic pregnancies.