Relationship between pre-pregnancy 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 pre-pregnancy BMI and gestational age (GA)-specific rates of stillbirth and perinatal death in women with chronic hypertension.

Objective: The objective of this study was to examine the relationship between pre-pregnancy BMI and GA-specific risk of stillbirth and perinatal death in the presence/absence of chronic hypertension.

Methods: This was a retrospective cohort study of all singleton births in the United States in 2016-17. 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 and perinatal death in women with and without chronic hypertension, adjusted for potential confounders. Results were expressed as GA-specific adjusted hazard ratios (aHR) and 95% confidence intervals (CI).

Results: A total of 7,365,797 women were included among whom 255,464 (3.5%) were underweight, 3,233,710 (43.9%) had normal BMI, 1,925,510 (26.1%) were overweight, and 1,065,958 (14.5%), 518,543 (7.0%), and 366,612 (5.0%) had obesity class I, II and III, respectively. Overall, stillbirth rates increased with increasing BMI and were higher in women with chronic hypertension (14.2 per 1000 total births) than among those without (4.7 per 1000 total births). The cumulative incidence of stillbirth increased at each gestational week, with the gradient increasing by BMI 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 BMI was reversed at 26-35 weeks' gestation. For example, at 31 weeks' gestation, the aHR for women with a BMI of 40 kg/m vs 20 kg/m and chronic hypertension was 0.78 (95% CI = 0.65, 0.93), while aHR for similar women without chronic hypertension was 1.39 (95% CI = 1.30, 1.48). Results were similar for perinatal death.

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