Pre-Pregnancy Body Mass Index, Diabetes Status, and Gestational Age-Specific Stillbirth Risk: A Nationwide Retrospective Cohort Study.

OBJECTIVE

To quantify and compare the gestational age (GA)-specific risks of stillbirth across pre-pregnancy body mass index (BMI) categories, stratified by pre-pregnancy diabetes status:

DESIGN

Retrospective population-based cohort study.

SETTING

United States from 2022 to 2023.

POPULATION OR SAMPLE: Singleton live and stillbirths between 20 and 43 weeks' gestation.

METHODS

Data were obtained from the live birth and fetal death certificates available from the National Center for Health Statistics. We used Piecewise Additive Mixed Models to assess the GA-specific relationship between pre-pregnancy BMI and stillbirth in women with and without pre-pregnancy diabetes mellitus adjusted for confounders. Results were expressed as gestational age-specific adjusted hazard ratios (aHR), and weekly risk estimates from 37 to 40 weeks' gestation.

MAIN OUTCOME MEASURES: Stillbirth.

RESULTS

A total of 6 923 146 women were included of which 187 734 (2.7%) were underweight, 2 631 390 (38.0%) had normal BMI, 1 915 636 (27.7%) were overweight, and 1 169 109 (16.9%), 591 078 (8.5%), and 428 199 (6.1%) had obesity class I, II and III, respectively. Overall, stillbirth rates increased with increasing BMI and were higher in women with pre-pregnancy diabetes (16.6 per 1 000 total births) than those without these conditions (4.4 per 1 000 total births). The gestational age-specific associations between elevated BMI and stillbirth differed depending on the presence of pre-pregnancy diabetes across all gestational weeks. For example, at 31 weeks' gestation, the aHR for women with a BMI of 40 kg/m2 vs. 20 kg/m2 and diabetes was 0.68 (95% CI = 0.54 to 0.85), while the same aHR for a non-diabetic pregnancy was 1.22 (95% CI = 1.13, 1.33). Absolute risks were highest in diabetic women with class III obesity.

CONCLUSIONS

Among obese women, the optimal gestational age for delivery depends on the presence of other risk factors such as pre-pregnancy diabetes.