Effect of Population-Specific Birthweight Curves on Disparities in Perinatal Mortality in Small-for-Gestational Age Pregnancies

Objective: To determine the effect of adopting sex or race/ethnicity-specific birthweight curves on small-for-gestational age (SGA)-associated mortality rates for specific populations.

Materials and methods: Analyzing 20,095,735 singleton pregnancies, we compared rates of perinatal death associated with SGA in distinct sex and racial/ethnic groups when SGA was defined using nonspecific, sex-specific, and race/ethnicity-specific birthweight curves.

Results: With use of a nonspecific birthweight curve, the rate of perinatal death was higher for SGA males (20.4/1,000 [95% confidence interval (CI), 20.1, 20.7]) than SGA females [14.6/1,000 (95% CI, 14.4, 14.8)]. With a sex-specific curve, this disparity was reduced, measuring 17.7/1,000 (95% CI, 17.4, 17.9) for SGA males and 16.3/1,000 (95% CI, 16.1, 16.6) for females. Using a nonspecific birthweight curve, perinatal death rates were higher for non-Hispanic blacks (20.4/1,000 [95% CI, 20.0, 20.8]) than for all other racial/ethnic groups (15.9/1,000 [95% CI, 15.7, 16.1]). This difference increased with use of a race-specific birthweight curve: perinatal mortality was 29.7/1,000 (95% CI, 29.0, 30.3) for SGA blacks and 14.7/1,000 (95% CI, 14.6, 14.9) for all other racial/ethnic groups.

Conclusion: Population-based differences in SGA-associated mortality are reduced with adoption of a sex-specific birthweight curve, but widen with use of a race/ethnicity-specific curve. These findings highlight the importance of outcomes analysis in the selection of diagnostic criteria for SGA.