Low and High Prepregnancy BMI Raise Maternal Mortality

Lara C. Pullen, PhD

November 14, 2017

Low and high prepregnancy body mass index (BMI) increase the risk for severe maternal morbidity and mortality (adjusted absolute risk increase of 28.8 per 10,000 for underweight and 61.1 per 10,000 for class 3 obese women), a study found.

Sarka Lisonkova, MD, PhD, assistant professor of maternal fetal medicine at the University of British Columbia in Canada, and colleagues designed their cohort study to determine whether prepregnancy BMI is associated with severe maternal morbidity. Their analysis of 743,630 pregnant women was published online November 14 in JAMA.

The team performed the research in Washington State between 2004 and 2013. Their study design allowed them to link vital statistics with discharge data.

During those 10 years, the investigators consistently collected a large amount of information on maternal characteristics and were able to analyze a wide variety of specific clinical conditions. Although information on maternal education, race, and BMI were self-reported, the researchers used International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) codes to document medical conditions.

The authors explain, however, that their use of ICD-9-CM codes meant that not all severely morbid conditions were captured. For example, they could not identify women with eclampsia superimposed on chronic hypertension.

The researchers also note the number of maternal deaths in their study was low relative to reports on US maternal mortality. Their study thus had insufficient statistical power to assess associations between BMI and rare severe morbidity subtypes and maternal death. They therefore created a composite outcome that included maternal death and conditions leading to serious sequelae.

The investigators documented that 3.2% of their population were underweight, 47.5% were normal weight, 25.8% were overweight, 13.1% could be classified as obese class I, 6.2% were obese class 2, and 4.2% were obese class 3.

Not only did pregnant women with low and high prepregnancy BMI have a statistically significant but small absolute increase in severe maternal morbidity and mortality relative to women with normal BMI, but the rates of severe maternal morbidity and mortality also revealed a dose-response pattern among women with above-normal BMI.

Specifically, the researchers calculated the absolute risk for severe maternal illness or death per 10,000 women for each BMI group relative to normal BMI. Compared with women with normal BMI, absolute risks per 10,000 women were 17.6 for overweight women, 24.9 for obese class 1 women, 35.8 for obese class 2 women, and 61.1 for obese class 3 women.

Obesity Pandemic

Although the medical community can successfully treat obesity-related illnesses, such as hypertension and diabetes, it is less able to treat or prevent obesity itself. The new research provides insights into one of the implications of the US obesity pandemic.

The new data are consistent with 2014 data revealing that approximately half of all pregnant women in the United States were overweight or obese. Although physicians have assumed that obesity did not bode well for the women, this study is the first to adequately document the risk of obesity to pregnant women.

The research also sheds more light on a category of women who are at increased risk for maternal morbidity. In an accompanying editorial, Aaron B. Caughey, MD, PhD, professor of obstetrics and gynecology at Oregon Health & Science University in Portland, calls on physicians and public health officials to move beyond educational and environmental modifications to reduce obesity and related morbidity and help these women at risk.

"As the study by Lisonkova et al indicates, high and low prepregnancy BMI levels were associated with small but statistically significant absolute increases in severe maternal morbidity," Dr Caughey writes. "Physicians and other clinicians who provide care for women of childbearing age should emphasize the importance of achieving optimal BMI for these women. But it is likely that public health efforts that include directed messages and innovative approaches will be required to help reverse the obesity pandemic."

The authors have disclosed no relevant financial relationships.

JAMA. Published online November 14, 2017. Abstract, Editorial

For more news, join us on Facebook and Twitter


Comments on Medscape are moderated and should be professional in tone and on topic. You must declare any conflicts of interest related to your comments and responses. Please see our Commenting Guide for further information. We reserve the right to remove posts at our sole discretion.