Relationship Between Pre-Pregnancy Body Mass Index and Gestational Age-Specific Risk of Stillbirth and Perinatal Death in Women With Chronic Hypertension.

Abstract

Obesity is an important risk factor for adverse birth outcomes. Its prevalence has steadily increased over the past decades, with nearly half of pregnant women in the United States considered overweight or obese in 2019. A higher pre-pregnancy body mass index (BMI) is associated with an elevated risk of stillbirth, and obesity is associated with chronic hypertension, diabetes mellitus, and other comorbidities that are independent risk factors for stillbirth and perinatal death. Because of these risks, earlier delivery for women with high BMI is being considered; however, the data on specific risks related to gestational age (GA) are limited. Currently, recommendations for women with high BMI differ across professional bodies. The Canadian Society of Obstetricians and Gynaecologists recommends delivery at 39 to 40 weeks' gestation for women with extreme obesity and at 38 to 40 weeks' gestation for those with chronic hypertension. In contrast, the American College of Obstetricians and Gynecologists and the Royal College of Obstetricians and Gynaecologists provide no GA-specific recommendations for obesity. The aim of this study was to evaluate the association between pre-pregnancy BMI and GA-specific risk of stillbirth, and whether chronic hypertension modifies these associations.This was a retrospective cohort study, using data from the National Center for Health Statistics on women who had a singleton live- or stillbirth at ≥20 weeks' gestation between 2016 and 2017. Excluded were live births and fetal deaths at <20 or ≥43 weeks of gestation and women with missing data on pre-pregnancy BMI. The primary and secondary outcomes were stillbirth and perinatal death, respectively. The primary exposure was pre-pregnancy BMI. BMI categories included underweight (BMI <18.5 kg/m2), normal weight (18.5 to <25 kg/m2), overweight (25 to <30 kg/m2), obesity class I (30 to <35 kg/m2), obesity class II (35 to <40 kg/m2), and obesity class III (≥40 kg/m2). Chronic hypertension was evaluated as a modifier on the relationship between BMI and GA-specific stillbirth.A total of 7,365,797 women were included in the study, with 3.5% classified as underweight, 43.9% as normal weight, 26.1% as overweight, 14.5% as obesity class I, 7% as class II, and 5% as class III. Women with elevated BMI had higher rates of chronic hypertension and pre-pregnancy diabetes mellitus. They were also more likely to deliver at an earlier GA. While the stillbirth rate among those at normal weight was 3.86 per 1000 total births, the rate increased with elevated BMI both with and without hypertension. Overall, women who also had chronic hypertension had a higher stillbirth rate in all BMI categories, with the highest rate of perinatal death among underweight women (27.6 per 1000 total births).Women without hypertension saw an increase in the GA-specific risk of stillbirth and perinatal death after 37 weeks of gestation. Regardless of whether hypertension was present, the differences in the hazard ratios (HR) curve remained consistent with increasing rates in the term period for both, but were higher among those with hypertension. When stratified by BMI subgroup, the highest rates were seen among those with Class III obesity and hypertension, but the trends of increasing risk at term were seen in all groups.In conclusion, the association between pre-pregnancy BMI and stillbirth is modified when chronic hypertension is present. Overall, the absolute risks of stillbirth and perinatal death at all gestations were higher in women with elevated BMI and chronic hypertension.(Abstracted from Am J Obstet Gynecol. 2025;233:61.e1-15.).