From the Journals

Age, race affect preterm birth risk in women with obesity



Risk for preterm birth in women with prepregancy obesity differs significantly by the mother’s age and race/ethnicity, according to new findings from an analysis that used a large, ethnically diverse population sample.

Previous study findings have demonstrated that pregnant women with obesity have a higher risk of giving birth to preterm babies, but the effect of age and race on that risk was not clear until now.

In this latest study, Wei Bao, MD, and colleagues at the University of Iowa, Iowa City, looked at records from 7.14 million live births registered in the U.S. National Vital Statistics System for 2016 and 2017, of which about 7.4% were preterm. The researchers excluded from their sample women with preexisting diabetes or hypertension.

For the cohort overall, there was a significant association between prepregnancy body mass index and preterm birth, with mothers who were overweight (adjusted odds ratio, 1.02; 95% confidence interval, 1.01–1.03) or obese (aOR, 1.18; 95%CI, 1.18–1.19), having a significantly higher risk of preterm birth, compared with healthy weight mothers. Underweight women also had a greater risk of preterm birth, compared with the healthy weight references (aOR, 1.33; 95% CI, 1.31–1.35), the researchers reported, adding that the association between maternal underweight and preterm birth was consistent across the maternal age and race/ethnicity groups.

Dr. Bao and colleagues found that, among non-Hispanic white women (who made up about half the cohort), maternal obesity was inversely associated with preterm birth when mothers were younger than 20 years (aOR, 0.92; 95% CI, 0.88-0.97), but there was a crossover effect at age 20, when maternal obesity became positively associated with preterm birth until age 39 (aOR, 1.04 at ages 20-24, to 1.40 at ages 35-39). A similar pattern was seen in Hispanic women, for whom maternal obesity was not associated with preterm birth when they were younger than 20 (aOR, 0.98; 95% CI, 0.93-1.04), but was positively associated with preterm birth after age 20 until age 39 (aOR, 1.06 at ages 20-24, to 1.38 at ages 35-39).

However, the crossover effect occurred considerably later in black women with obesity, for whom maternal obesity remained inversely associated with preterm birth until age 30 (aOR, 0.76 before age 20; 0.83 at ages 20-24; 0.98 at ages 25-29), at which point the crossover effect kicked in, and maternal obesity became positively associated with preterm birth, increasing steadily with advancing age (aOR, 1.15 at ages 30-34; 1.26 at ages 35-39; 1.29 from age 40). “Our results, which are based on a large and diverse U.S. population, provide, for the first time, a comprehensive review of the association between maternal obesity and preterm birth for women [at a] range of ages,” Dr. Bao and colleagues wrote in their analysis, which was published in Lancet Diabetes & Endocrinology.

The researchers hypothesized that the inverse association between prepregnancy obesity and preterm birth in teenagers and younger women could be explained by the fact that “[healthy weight] teenagers, who are still growing and developing, might compete with the fetus for nutrients, which could subsequently affect physiological and metabolic systems involved with parturition,” whereas pregnant teenagers with obesity “might not need to compete (or compete to a lesser extent) for nutrients with their babies for their own growth.” The researchers stressed that more research was needed to understand the underlying mechanisms of the associations. The findings of a protective effect until age 30 in black women also require further study, Dr. Bao and colleagues said.

They stressed that the findings do not argue for weight gain as a preventive measure against preterm birth for normal weight young women, as “younger women, whether obese or not, have a higher risk of preterm birth than women aged 25-29 years do in Hispanic and in non-Hispanic white populations. Additionally, the adverse effects that maternal obesity has on other perinatal and neonatal outcomes should not be overlooked.”

The National Institutes of Health funded the study. The authors declared no conflicts of interest.

SOURCE: Bao et al. Lancet Diabetes Endocrinol. 2019;7:707-14.

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