Prepregnancy Obesity Linked to Earliest Preterm Births

Laurie Barclay, MD

June 27, 2014

Obese women who become pregnant are more likely to deliver before 28 weeks of pregnancy?, but the association between prepregnancy body mass index (BMI) and risk for preterm birth is complex and affected by race/ethnicity, gestational age, and parity, according to findings of a population-based cohort study published in the July issue of Paediatric and Perinatal Epidemiology.

"Until now, people have been thinking about preterm birth as one condition, simply by defining it as any birth that happens at least three weeks early," lead author Gary Shaw, DrPH, professor of pediatrics at Stanford University School of Medicine in Palo Alto, California, said in a news release. "But it's not as simple as that. Preterm birth is not one construct; gestational age matters."

Using a cohort of 989,687 singleton births in California from 2007 to 2009, the investigators examined associations between prepregnancy BMI and spontaneous preterm birth over a range of BMI, gestational age, and race/ethnicity. They excluded women with pregestational diabetes, gestational diabetes, gestational hypertension, preeclampsia/eclampsia, and chronic hypertension.

After adjustment for maternal age, education, height, and prenatal care initiation, prepregnancy obesity (BMI ≥ 30.0 kg/m2) in first-time mothers was associated with a dramatically increased risk for delivery before 28 weeks, but not at later gestational ages. For each race/ethnic group, risks were highest at the earliest gestational ages and at the highest levels of obesity.

For first-time mothers in the obese III category (BMI ≥ 40.0 kg/m2), relative risks for preterm birth at 20 to 23 weeks were 6.29 (95% confidence interval [CI], 3.06 - 12.9] for non-Hispanic whites, 4.34 (95% CI, 2.30 - 8.16) for non-Hispanic blacks, and 4.45 (95% CI, 2.53 - 7.82) for Hispanics.

For obese women in their second or subsequent pregnancy, risk for delivery at 20 to 23 weeks was higher than in normal-weight women, but the relative risk was less elevated than in obese first-time mothers.

Regardless of parity or race/ethnicity, underweight (BMI < 18.5 kg/m2) was associated with slightly elevated risks for preterm birth at 24 or more weeks.

Limitations of this study include data derived from birth certificates and discharge databases.

"Ideally, as the American College of Obstetricians and Gynecologists has already recommended, women should embark upon a healthy diet and exercise program before becoming pregnant," study coauthor Deirdre Lyell, MD, associate professor of obstetrics and gynecology at Stanford and an obstetrician at Lucile Packard Children's Hospital Stanford, said in the news release.

"Women who are obese should also consider meeting their obstetrician to discuss and understand the risks beyond those identified in this research," she added.

Additional risks for obesity include higher rates of gestational diabetes, preeclampsia, and cesarean delivery.

Despite the known increased risk for early preterm delivery in black women, race/ethnicity did not explain the effect of obesity on early preterm delivery in this study.

"This is a clue that informs what sort of research we should try next," Dr. Shaw said in the release. "We believe the inflammatory process is contributing to preterm birth, particularly at earlier gestational ages. We're at the edge of trying to solve this puzzle."

The March of Dimes Prematurity Research Center at Stanford University School of Medicine funded this study. The authors have disclosed no relevant financial relationships.

Paediatric Perinatal Epidemiol. 2014;28:302-311. Abstract

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