August 22, 2019
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Maternal age, race mediate association between prepregnancy obesity, preterm birth

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An observed association between prepregnancy obesity and preterm birth varies based on age and race, with black women less likely to deliver preterm at age 30 years vs. white and Hispanic women, according to a large population-based study.

Buyun Liu

Maternal prepregnancy obesity is significantly associated with the risk of preterm birth in the general population, but the risk differs according to maternal age and race or ethnicity,” Buyun Liu, MD, PhD, a postdoctoral research scholar in the department of epidemiology at the University of Iowa College of Public Health, told Endocrine Today. “Our findings suggest that age- and race-specific recommendations for pregnant women are needed in counseling the risk for preterm birth during prenatal care. However, further investigation is needed to replicate our findings, assess the causality and determine the underlying mechanisms.”

Liu and colleagues analyzed birth certificate data from the U.S. National Vital Statistics System for 2016 and 2017 for 7,141,630 mothers with a live singleton birth who did not have preexisting hypertension or diabetes. Prepregnancy obesity was defined as a prepregnancy BMI of at least 30 kg/m². Preterm birth was defined as gestational age of less than 37 weeks. Researchers used logistic regression analysis adjusted for maternal age and race to estimate ORs for preterm birth.

Within the cohort, 527,637 births (7.4%) were preterm (10.4% black mothers; 7.5% Hispanic mothers; 6.6% white mothers).

Age, racial differences

In the overall population, mothers with obesity were more likely to deliver preterm vs. mothers with normal weight, with an adjusted OR of 1.18 (95% CI, 1.18-1.19). Maternal underweight was also positively associated with increased risk for preterm birth vs. normal weight (aOR = 1.33; 95% CI, 1.31-1.35), with researchers observing a consistency across maternal age groups and racial groups. However, researchers found that the association between prepregnancy obesity and preterm birth risk varied in analyses stratified by maternal age and race (P for interaction < .0001).

Pregnant women in hospital 
An observed association between prepregnancy obesity and preterm birth varies based on age and race, with black women less likely to deliver preterm at age 30 years vs. white and Hispanic women.
Source: Adobe Stock

“Stratified analysis showed that the association between prepregnancy obesity and preterm birth was inverse among women younger than 25 years and among non-Hispanic black women,” the researchers wrote. “Therefore, we did further analyses by age [and] race or ethnicity and noted a crossover effect of age in the association between prepregnancy obesity and preterm birth.”

Among white women, maternal obesity was inversely associated with preterm birth among those younger than 20 years (aOR = 0.92; 95% CI, 0.88-0.97); however, the association was positive among white women with obesity aged at least 20 years, with ORs ranging from 1.04 among women aged 20 to 24 years (95% CI, 1.01-1.06) to 1.39 among women aged at least 40 years (95% CI, 1.31-1.46).

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Among Hispanic women, maternal obesity was similarly not associated with preterm birth among women younger than 20 years (OR = 0.98; 95% CI, 0.93-1.04), but positively associated with preterm birth among women aged at least 20 years. However, among black women, maternal obesity was inversely associated with preterm birth among those younger than 30 years (OR = 0.76; 95% CI, 0.71-0.81), but positively associated with preterm birth among women aged at least 30 years, with ORs ranging from 1.15 for women aged 30 to 34 years (95% CI, 1.11-1.19) to 1.29 for women aged at least 40 years (95% CI, 1.18-1.42).

“It is worth noting that we should not recommend pregnant women gain weight to prevent preterm birth, although we found that higher prepregnancy BMI is associated with lower risk of preterm birth among young women,” Liu said.

‘Surprising’ findings

In commentary accompanying the study, Emily Oken, MD, MPH, a professor in the department of population medicine at Harvard Medical School and the Harvard Pilgrim Health Care Institute in Boston, and colleagues called the variation in preterm birth risk by race “surprising.”

“How could obesity confer benefit?” Oken and colleagues wrote. “It is plausible that young women with obesity might not yet have experienced any cardiometabolic sequelae, such as hypertension or diabetes, which are likely to mediate relationships with adverse pregnancy outcomes. Also, compared with non-Hispanic white women, non-Hispanic black women tend to be metabolically healthier at any given BMI.”

Oken and colleagues noted that an alternative explanation is that bias affects the results, adding that a limitation common to many studies that aim to estimate the causal effect of a prepregnancy exposure on a birth outcome is the challenge of competing events, such as failure to conceive or miscarriage.

“Potential unmeasured common causes of these events, such as genetic differences or social stressors, preclude live births and are associated with preterm delivery,” Oken and colleagues wrote. “Alternative analytic approaches could allow unbiased estimation of causal effects. These different approaches answer different causal questions and vary in terms of the strength of assumptions required to estimate these relations in real-world studies.” – by Regina Schaffer

For more information:

Buyun Liu, MD, PhD, can be reached at the University of Iowa College of Public Health, Department of Epidemiology, 145 N. Riverside Drive, Room S431 CPHB, Iowa City, IA 52242; email: buyun-liu@uiowa.edu.

Disclosures: The NIH funded this study. The authors report no relevant financial disclosures. Oken reports no relevant financial disclosures.