Babies born to women who have undergone bariatric surgery are more likely to be premature or small for their gestational age, according to a large registry study conducted at Karolinska Institutet in Sweden and published in the BMJ. Researchers believe that such pregnancies should be considered high risk and prenatal care should monitor them extra closely.

The number of bariatric operations performed has increased dramatically in recent years. Most of these operations are performed on women, and as a result the number of babies born to women who have undergone bariatric surgery is also increasing. Researchers at Karolinska Institutet have examined how the operation affects pregnancy. The study was based on data from the Swedish Medical Birth Register and the Patient Register.

The study compared over 2,500 babies born between 1992 and 2009 to women who had undergone bariatric surgery with 12,500 babies born to mothers who had not had the surgery. Pregnancies were matched individually, so that the mothers’ BMI, age, educational background, smoking habits, and previous births were comparable between the groups.

The researchers found that infants of women with bariatric surgery history have lower weights at delivery. 5.2 percent of them were small for gestational age and were at least two standard deviations below the normal as opposed to only 3 percent in the control group. Moreover, 4.2 percent of the babies of mothers with bariatric surgery history were large for gestational age, compared to 7.3 percent of the control group, and more were born prematurely: 9.7 percent before the 37th week, compared with 6.1 percent of the control group. The researchers also found no difference regarding stillbirth or neonatal death (within the first 27 days) between the two groups.

“Mothers with the same BMI gave birth to babies of varying weights depending on whether or not they had undergone bariatric surgery, so there is some kind of association between the two,” says Dr. Olof Stephansson, obstetrician and Associate Professor at the Clinical Epidemiology Unit at Karolinska Institutet. “The mechanism behind how surgery influences fetal growth we don’t yet know, but we do know that people who have bariatric surgery are at increased risk of micronutrient deficiencies.”

Researchers believe that women who have undergone bariatric surgery should be considered a risk group during pregnancy. Researchers also point out that bariatric surgery has numerous benefits for mothers, such as lowering the risk of diabetes, cardiovascular disease, cancer and stroke. Untreated obesity is a known risk factor for mothers and babies during pregnancy and childbirth.