WASHINGTON, DC—The largest study of obesity during pregnancy and babies with heart defects in the US finds that women who were overweight or obese before they became pregnant had an approximately 18% increased risk of having a baby with certain heart defects compared with women who were of normal body mass index (BMI) before they became pregnant. Severely obese women had approximately a 30% increased risk.

The CDC study, “Association Between Prepregnancy Body Mass Index and Congenital Heart Defects,” published in the American Journal of Obstetrics and Gynecology, found a significant increase in several types of heart defects in babies born to overweight and obese women, compared to babies born to normal weight women. These included obstructive defects on the right side of the heart, and defects in the tissue that separates the two upper chambers of the heart

The analysis included 6,440 infants with congenital heart defects and 5,673 infants without birth defects whose mothers were interviewed as part of the National Birth Defects Prevention Study (NBDPS). Obesity and overweight levels were determined based on the study’s participants’BMIs. Overweight is defined as a BMI of 25-29.9, moderate obesity is defined as a BMI 30-34.9, and severe obesity is defined as a BMI of 35 or higher.

The study looked at 25 types of heart defects and found associations with obesity for 10 of them. Five of these 10 types were also associated with being overweight before pregnancy. Women who were overweight, but not obese, had approximately a 15% increased risk of delivering a baby with certain heart defects. “Congenital heart defects are the most common types of birth defect, and among all birth defects, they are a leading cause of illness, death, and medical expenditures,” said Dr Edwin Trevathan, director of the CDC’s National Center on Birth Defects and Developmental Disabilities, in a statement. “Women who are obese and who are planning a pregnancy could benefit by working with their physicians to achieve a healthy weight before pregnancy.”

The study accounted for several important factors such as maternal age and race–ethnicity. Mothers with type 1 or 2 diabetes before they got pregnant, a strong risk factor for heart defects, were excluded from the study.

One important limitation of the study is that BMI is calculated based on self–reported weight and height, and weight may be underreported by women during the study interview. Although the study found an association between overweight and obesity and the risk of certain birth defects, further study is needed to determine whether body weight is the direct cause of these birth defects.

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