Hidden overweight and adverse birth outcomes among Asian mothers in the United States, 2016-2023: a population-based repeated cross-sectional study

Background: Standard body mass index (BMI) categories may understate risk in Asian individuals. Lower Asian action points exist, but their relevance to birth outcomes is unclear. Objectives: To estimate the proportion of normal-weight Asian mothers with BMI 23.0-24.9 kg/m2 and its association with birth outcomes, overall and by Asian subgroup, and to compare Asian and White mothers. Methods: Repeated cross-sectional study of US singleton live births, 2016-2023 (National Vital Statistics System). Outcomes were hypertensive disorders, caesarean delivery, preterm birth, and low birthweight. Modified Poisson regression, adjusted for confounders and Asian subgroup, estimated risk ratios (RRs), standardized risks, and risk differences (RDs). We tested for a change in slope at 23.0 kg/m2, subgroup heterogeneity, and race x BMI interaction. Results: Among 1,758,116 non-Hispanic Asian mothers, 314,510 (17.9%) had BMI 23.0-24.9 kg/m2, 30.4% of normal-weight mothers. Compared with BMI 18.5-22.9, standardized risk of hypertensive disorders was 4.5% versus 3.2% (RD 1.4 percentage points, 95% CI 1.3, 1.5; RR 1.44, 95% CI 1.40, 1.47), and of caesarean delivery 32.4% versus 28.6% (RR 1.14, 95% CI 1.13, 1.14). Preterm birth risk was 7.2% versus 6.6% (RR 1.08, 95% CI 1.06, 1.10); low birthweight risk was not higher (7.1% versus 7.5%; RR 0.95, 95% CI 0.93, 0.97). The hypertensive-disorder slope showed no evidence of change at 23.0 (ratio of per-unit RRs 0.99, 95% CI 0.98, 1.01); the hypertensive-disorder RR ranged from 1.24 (95% CI 1.18, 1.29) among Asian Indian to 1.70 (95% CI 1.60, 1.81) among Chinese mothers. BMI-outcome associations differed between Asian and White mothers. Conclusions: Nearly one in three normal-weight Asian mothers had BMI 23.0-24.9 kg/m2, associated with higher risks of hypertensive disorders and caesarean delivery but not low birthweight. The hypertensive-disorder gradient showed no evidence of change at 23.0 kg/m2, and associations varied across Asian subgroups.