Prepregnancy BMI Tied to Risk for Poor Maternal-Fetal Outcomes

Veronica Hackethal, MD

May 07, 2019

Prepregnancy body mass index (BMI) may be a more important risk factor for adverse maternal-fetal outcomes than weight gain during pregnancy, according to results of a meta-analysis published online today in JAMA.

"Results from this study suggest that maternal prepregnancy BMI was more strongly associated with adverse maternal and infant outcomes than gestational weight gain. Therefore, prepregnancy BMI may be an important focus for preconception counselling," write Ellis Voerman, MSc, from the Erasmus Medical Center, Rotterdam, the Netherlands, and colleagues with the LifeCycle Project-Maternal Obesity and Childhood Outcomes Study Group.

The researchers also calculated estimates of optimal gestational weight gain based on prepregnancy BMI. The estimates have the potential to be used during prenatal counseling, but were limited in their ability to predict adverse outcomes.

While guidelines for weight gain during pregnancy exist from the National Academy of Medicine (NAM), past studies have suggested room for improvement. Notably, the NAM guidelines do not include separate recommendations for very obese women. 

Weight gain is part of a healthy pregnancy. But too much gain in maternal fat stores has been linked to a number of adverse outcomes for both mother and child. Yet, about 47% of pregnant women in the United States gain more than the recommended amount of weight during pregnancy, according to a recent review.

Whereas many studies have looked at the effect of gestational weight gain on maternal-fetal outcomes, fewer have included prepregnancy BMI.

Therefore, Voerman and colleagues conducted a meta-analysis of 25 cohort studies conducted in Europe and North America. The researchers pooled individual patient data from 196,670 women with singleton live births between 1989 and 2015. Among participants, 20.8% were white, and the median age was 30 years.

They stratified participants by prepregnancy BMI categories using WHO definitions: underweight (4.0% of participants; BMI, < 18.5), normal weight (68.0%; BMI, 18.5-24.9), overweight (19.7%; BMI, 25.0-29.9), obesity grade 1 (6.1%; BMI, 30.0-34.9), obesity grade 2 (1.7%; BMI, 35.0-39.9), and obesity grade 3 (0.5%; BMI, ≥ 40.0).

Overall, 37.2% of women experienced any adverse outcome, which included one or more of the following: preeclampsia, gestational hypertension, gestational diabetes, cesarean delivery, preterm birth before 37 weeks, and small or large size for gestational age at birth.

Adverse outcomes generally increased with increasing prepregnancy BMI, occurring in 34.7% of underweight women, 34.1% of normal-weight women, 42.0% of women with overweight, and 50.2%, 56.8%, and 61.1% of women with obesity grades 1, 2, and 3, respectively.

When the authors stratified participants by prepregnancy BMI and then examined gestational weight gain, they found that the risk for any adverse outcome increased with increasing prepregnancy BMI and was largely independent of gestational weight gain.

Overall, the lowest risk for any adverse event (26.7%) was among women with a BMI below 18.0 and gestational weight gain of 26.0 kg to 27.9 kg.

Among normal-weight women, the risk for any adverse outcome was lowest at 29.2% with gestational weight gain between 14.0 kg and 15.9 kg and highest at 50.2% with gestational weight gain of less than 8.0 kg.

Among women with overweight, the risk ranged from 37.3% with gestational weight gain of 2.0 kg to 3.9 kg to 56.4% with gestational weight gain of 28.0 kg or greater.

For women with obesity grade 1, 2, or 3, risk increased with increasing gestational weight gain. The highest risks were 63.7% for gestational weight gain of 28.0 kg or greater among those with obesity grade 1, 67.7% for weight gain of 16.0 kg or greater among those with obesity grade 2, and 78.8% for gestational weight gain of 16.0 kg or more among those with obesity grade 3.

"The association of maternal prepregnancy BMI with the risk for any adverse outcomes was stronger than the association of gestational weight gain," the authors write. "The [odds ratios] for the risk of any adverse outcome were 1.28 (95% [confidence interval (CI)], 1.27-1.29) and 1.04 (95%CI, 1.03-1.05) per 1-[standard deviation] increase in maternal prepregnancy BMI and gestational weight gain, respectively (<.001 for comparison)."

Based on their findings, the authors estimated the optimal gestational weight gain for each prepregnancy BMI category, with amounts ranging from 0 to 17.9 kg.

They note that their optimal gestational weight gain estimates are roughly similar to NAM recommendations for under- and normal-weight women, but lower for all categories of obesity.

However, they caution that this study included few very obese women, which could have biased results. "Future studies should evaluate the effect and safety of weight loss during pregnancy in severely obese women," they write.

Nevertheless, they conclude: "The findings from this study suggest that prepregnancy weight might be a more important target for interventions than gestational weight gain."

In an accompanying editorial, Mary M. McDermott, MD, from Northwestern University Feinberg School of Medicine, Chicago, Illinois, and Linda Brubaker, MD, from the University of California San Diego, La Jolla, concur with that finding.

"[A]n important conclusion of the report by Voerman et al is that prepregnancy BMI was more strongly associated with adverse maternal and infant outcomes than the amount of gestational weight gain," they write.

"Based on the study by Voerman et al, resources should be dedicated toward ensuring an optimal BMI for all women of reproductive age rather than on gestational weight gain. Recent guidelines and available services can help achieve this important public health goal."

The authors mention several study limitations. Gestational weight gain is nonlinear, and the study could not evaluate how gestational weight gain affects outcomes over time. In addition, most data on prepregnancy BMI was self-reported, and the analysis could not evaluate outcome severity or stillbirth.

One or more authors report speaker fees, institutional funding, and/or other support from one or more of the following: nutritional product companies, Abbott Nutrition, Nestec, Danone, Roche Diagnostics and/or Medtronic.

McDermott reports funding from ViroMed, Hershey Company, Chromadex, and Reserveage; and grants from Regeneron. Brubaker reports personal fees from the Female Pelvic Medicine and Reconstructive Surgery and UpToDate.

JAMA. 2019;321:1702-1715. Published online May 7 2019. Full text

JAMA. 2019;321:1715. Published online May 7 2019. Full text

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