Relationship between prepregnancy body mass index and gestational age-specific risk of stillbirth and perinatal death in women with chronic hypertension.

Background: Obesity is a risk factor for stillbirth and perinatal death and is often accompanied by chronic hypertension. However, there are few studies on the relationship between prepregnancy body mass index and gestational age-specific risks of stillbirth and perinatal death in women with chronic hypertension.

Objective: This study aimed to examine the relationship between prepregnancy body mass index and gestational age-specific risks of stillbirth and perinatal death in the presence/absence of chronic hypertension.

Study Design: This was a retrospective cohort study of all singleton births in the United States from 2016 to 2017. Data were obtained from the live birth and fetal death certificates available from the National Center for Health Statistics. Piecewise additive mixed models were used to assess the relationship between prepregnancy body mass index and gestational age-specific risks of stillbirth and perinatal death in women with and without chronic hypertension, adjusted for potential confounders. Results were expressed as gestational age-specific adjusted hazard ratios and 95% confidence intervals.

Results: A total of 7,365,797 women were included in the study, of whom 255,464 (3.5%) were underweight; 3,233,710 (43.9%) had normal body mass index; 1,925,510 (26.1%) were overweight; and 1,065,958 (14.5%), 518,543 (7.0%), and 366,612 (5.0%) had obesity classes I, II, and III, respectively. Overall, stillbirth rates increased with increasing body mass index and were higher in women with chronic hypertension (14.2 per 1000 total births) than in those without chronic hypertension (4.7 per 1000 total births). The cumulative incidence of stillbirth increased at each gestational week, with the gradient increasing by body mass index category in women without chronic hypertension. However, this relationship was modified in women with chronic hypertension, for whom the increased risk of stillbirth by higher body mass index was reversed at 26 to 35 weeks of gestation. For example, at 31 weeks of gestation, the adjusted hazard ratio of women with a body mass index of 40 kg/m vs those with a body mass index of 20 kg/m and chronic hypertension was 0.78 (95% confidence interval, 0.65-0.93), whereas the adjusted hazard ratio of similar women without chronic hypertension was 1.39 (95% confidence interval, 1.30-1.48). The results were similar for perinatal death.

Conclusion: The relationship between prepregnancy body mass index and stillbirth is modified in the presence of chronic hypertension at 26 to 35 weeks of gestation when elevated body mass index is associated with a lower or similar relative risk of stillbirth and perinatal death. Nevertheless, women with chronic hypertension and elevated body mass index have higher absolute risks of stillbirth and perinatal death at all gestations. Our results suggest that, in women with obesity, optimal timing of delivery may differ depending on the presence or absence of chronic hypertension.