Maternal Prepregnancy Body Mass Index and Risk of Spontaneous Preterm Birth

Abstract

Background: Findings from studies examining risk of preterm birth associated with elevated prepregnancy body mass index (BMI) have been inconsistent. Methods: Within a large population-based cohort, we explored associations between prepregnancy BMI and spontaneous preterm birth across a spectrum of BMI, gestational age, and racial/ethnic categories. We analysed data for 989 687 singleton births in California, 2007-09. Preterm birth was grouped as 20-23, 24-27, 28-31, or 32-36 weeks gestation (compared with 37-41 weeks). BMI was categorised as <18.5 (underweight); 18.5-24.9 (normal); 25.0-29.9 (overweight); 30.0-34.9 (obese I); 35.0-39.9 (obese II); and ≥40.0 (obese III). We assessed associations between BMI and spontaneous preterm birth of varying severity among non-Hispanic White, Hispanic, and non-Hispanic Black women. Results: Analyses of mothers without hypertension and diabetes, adjusted for age, education, height, and prenatal care initiation, showed obesity categories I-III to be associated with increased risk of spontaneous preterm birth at 20-23 and 24-27 weeks among those of parity 1 in each race/ethnic group. Relative risks for obese III and preterm birth at 20-23 weeks were 6.29 [95% confidence interval (CI) 3.06, 12.9], 4.34 [95% CI 2.30, 8.16], and 4.45 [95% CI 2.53, 7.82] for non-Hispanic Whites, non-Hispanic Blacks, and Hispanics, respectively. A similar, but lower risk, pattern was observed for women of parity ≥2 and preterm birth at 20-23 weeks. Underweight was associated with modest risks for preterm birth at ≥24 weeks among women in each racial/ethnic group regardless of parity. Conclusions:The association between women's prepregnancy BMI and risk of spontaneous preterm birth is complex and is influenced by race/ethnicity, gestational age, and parity.

Spontaneous and indicated preterm delivery risk is increased among overweight and obese women without prepregnancy chronic disease

“…Our finding of increased risk of spontaneous preterm delivery at 23–27 weeks among obese nulliparous women is similar to a previous large California study, 31 based on birth records linked with hospital discharge data that excluded chronic as well as gestational hypertension and diabetes and stratified by parity, and another Swedish study 21 based on birth registry that included women with chronic diseases. Shaw et al 31 also reported an inverse association between excess prepregnancy BMI and the risk of spontaneous moderate to late preterm delivery which is similar to our results.…”

“…11, 12, 31 Among studies which reported overall risk of spontaneous preterm deliveries in association with prepregnancy obesity, some reported null associations, 30 or an increased overall risk among obese women, 21, 23 whereas others reported a decreased risk. 11, 25, 29 Difference in preterm delivery incidence, case classification, racial composition, and study design could partly explain these inconsistencies.…”

Intrapartum Management of the Obese Gravida

“…In a cohort of nearly 1 million singleton births in California between 2007 and 2009, maternal obesity was associated with a significantly increased risk for spontaneous PTB at 20 to 23 weeks and 24 to 27 weeks across all racial/ethnic groups. 6 Obesity was not consistently associated with later PTB, however.…”

Placenta: The Forgotten Organ

“…Recent epidemiological studies also suggest that derangements in glucose and fatty acid metabolism may drive such pregnancy complications. For example, maternal gestational diabetes mellitus (GDM) and obesity are independently, and additively, associated with increased rates of PE (Catalano et al 2012) as well as spontaneous preterm birth (Shaw et al 2014). There may be shared mechanisms contributing to PE and preterm labor (PTL) pathogenesis (see below).…”