(495) Association between body-mass-index and gestational-age-specific rates of stillbirth in women with and without chronic hypertension

Body-mass-index (BMI) specific recommendations to inform optimal timing of delivery are scarce. This study describes gestational age-specific risks of stillbirth in women with varying pre-pregnancy BMI and assesses how these risks change in the presence of chronic hypertension (CH).

 

Study Design:

This was a retrospective cohort study of all singleton births in the United States, 2016-2017. Rates of stillbirth per 1000 total births were compared by between BMI categories, stratified by CH. Piecewise additive mixed models were used to estimate the association between pre-pregnancy BMI and stillbirth and possible effect modification by CH. Models were adjusted for relevant factor (e.g., age, smoking etc). These models were also used to compare gestational age specific risks using a fetuses-at-risk approach. 

Results:

The study included 7,363,952 women with singleton birth; 254,225 (3.5%) were underweight, 3,220,432 (43.9%) had normal BMI, 1,918,480 (26.1%) were overweight and 1,062,177 (14.4%), 516,693 (7.0%), 365,357 (5.0%) had class I, II and III obesity, respectively. Rates of stillbirth increased from 3.85 per 1000 births in women with normal BMI to 8.26 per 1000 births in women with obesity class III. Women with and without CH had 14.2 and 4.7 stillbirths per 1000 total births, respectively.  Average joint effects of hypertension and obesity led to 3 to 5 times increased rates compared with women with normal BMI who were normotensive (e.g., adjusted hazard ratio for women with obesity class III and hypertension was 4.68, 95% CI 3.73 - 5.86). These increases in rates of stillbirth varied across gestational age (p < 0.001), with sharper increases at late term and post-term gestation, especially in women with hypertension.

Conclusion:

Overweight and obese women are at increased risk of stillbirth, and this risk is compounded by concurrent chronic hypertension. Gestational age-specific differences in the risk of stillbirth increased with gestational age, suggesting an optimal timing of delivery at early term gestation in women with obesity accompanied by chronic hypertension.