Highlights
1. Stillbirth rates increase with rising pre-pregnancy BMI, peaking in obese women with pre-existing diabetes (16.6 vs. 4.4 per 1,000 births in non-diabetics).
2. At 31 weeks, BMI 40kg/m² diabetic pregnancies show paradoxically lower risk (aHR 0.68) versus non-diabetics (aHR 1.22), suggesting distinct pathophysiology.
3. Absolute stillbirth risks are highest in class III obese diabetics, demanding careful monitoring and individualized delivery planning.
Background
Maternal obesity affects 30% of US pregnancies and doubles stillbirth risk, while pregestational diabetes compounds this risk through placental dysfunction and metabolic dysregulation. Current guidelines lack granularity on gestational age-specific risks across BMI-diabetes subgroups, potentially delaying interventions for highest-risk pregnancies.
Study Design
This retrospective cohort analyzed 6.9M US singleton births (2022-2023) via National Center for Health Statistics data. Piecewise Additive Mixed Models generated adjusted hazard ratios (aHRs) for stillbirth across BMI categories (underweight to class III obesity), stratified by diabetes status and weekly gestation periods (20-43 weeks). Models controlled for maternal age, race, smoking, and hypertension.
Key Findings
Population Characteristics
The cohort comprised 6.9M women: 2.7% underweight, 38% normal BMI, 27.7% overweight, and 31.5% with obesity (16.9% class I, 8.5% class II, 6.1% class III). Diabetic women (2.1% prevalence) had 3.8-fold higher stillbirth rates.
Risk Patterns
The study revealed three critical patterns: 1) Non-linear risk curves with accelerated risk progression after 37 weeks in obese non-diabetics; 2) Divergent BMI effects in diabetics showing U-shaped risk relationships; 3) Absolute risk thresholds exceeding 1/100 births by 39 weeks in class III obese diabetics.
Time-Sensitive Interventions
Optimal delivery windows varied by BMI-diabetes status: Class III obese diabetics reached risk parity with average-risk pregnancies 2-3 weeks earlier than obese non-diabetics, suggesting potential benefit from earlier delivery in this subgroup.
Expert Commentary
“These findings underscore the need for dual-axis risk assessment incorporating both metabolic status and BMI,” notes Dr. Sarah Kilpatrick, Chair of Obstetrics at Cedars-Sinai. The paradoxical protective aHR in mid-gestation diabetics may reflect heightened surveillance, while methodological limitations include potential underascertainment of stillbirths early in gestation.
Conclusion
This study provides the first granular risk stratification for stillbirth across BMI-diabetes combinations, supporting earlier delivery consideration for obese diabetics while avoiding unnecessary interventions in lower-risk subgroups. Future research should validate these patterns in prospective cohorts and evaluate targeted antenatal testing strategies.
Funding
National Institutes of Health R01-HD098187