Risks for Preterm Births May Be Higher Among Overweight and Obese Mothers

Laurie Barclay, MD

July 28, 2010

July 28, 2010 — Risks for preterm births may be higher among overweight and obese mothers, according to the results of a systematic review and meta-analyses reported in the July 20 issue of the BMJ.

The goal of the study was to examine the association of maternal overweight and obesity with preterm birth and low birth weight in singleton pregnancies in developed as well as in developing countries. Sarah D. McDonald, from McMaster University in Hamilton, Ontario, Canada, and colleagues searched MEDLINE and EMBASE from their beginnings, as well as bibliographies of retrieved articles. Inclusion criteria were studies of the effect of overweight and obesity vs a reference group of women with normal body mass index (BMI), on 2 main study endpoints of preterm birth (< 37 weeks) and low birth weight (< 2500 g).

Using a piloted data collection form, 2 investigators independently reviewed titles, abstracts, and full articles; extracted information; and evaluated the quality of the retrieved studies. The 84 studies included in the meta-analyses enrolled a total of 1,095,834 women. Of these studies, 64 were cohort studies and 20 were case-control studies.

Compared with women of normal weight, overweight and obese women had a similar risk for preterm birth overall but an increased risk for induced preterm birth (relative risk [RR], 1.30; 95% confidence interval [CI], 1.23 - 1.37) and a lower risk of having an infant of low birth weight (RR, 0.84; 95% CI, 0.75 - 0.95). The reduction in the risk of having an infant of low birth weight was greater in developing countries vs developed countries (RR, 0.58; 95% CI, 0.47 - 0.71 vs RR, 0.90; 95% CI, 0.79 - 1.01).

Analyses to account for publication bias showed that when imputed "missing" studies were added, the apparent protective effect of overweight and obesity on low birth weight disappeared, whereas the risk for preterm birth appeared significantly higher in overweight and obese women (RR, 1.24; 95% CI, 1.13 - 1.37).

Compared with normal-weight women, very obese women were at 70% greater risk for induced preterm birth before 37 weeks and at 82% greater risk for early preterm birth (before 32 or 33 weeks).

"Overweight and obese women have increased risks of preterm birth and induced preterm birth and, after accounting for publication bias, appeared to have increased risks of preterm birth overall," the study authors write. "The beneficial effects of maternal overweight and obesity on low birth weight were greater in developing countries and disappeared after accounting for publication bias."

Limitations of this review include potential residual confounding and inability to determine causal relationships or underlying mechanisms.

"Future research is needed to try to determine why overweight and obese women are at risk of preterm birth, and to determine effective methods of weight loss in women of childbearing age before pregnancy," the study authors conclude. " ...Clinicians need to be aware that overweight or obesity in women is not protective against having infants of low birth weight and should consider surveillance when indicated. Ideally, overweight or obese women should have prepregnancy counselling so that they are informed of their perinatal risks and can try to optimise their weight before pregnancy."

The Canadian Institute of Health Research supported this study and 2 of its authors. A third study author was supported by the China Scholarship Council.

BMJ. 2010;341:c3428. Abstract

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