Gastric bypass surgery in cases of women with severe obesity before they become pregnant leads to less birth defects in their babies, a new study in Sweden suggests.

Overall, 3.4% of women who had surgery had babies with major birth defects, compared with 4.9% of women who didn’t have weight-loss operations, reports the study in JAMA.

Changes in weight and blood sugar may explain these results, said to Reuters lead study author Martin Neovius, from the Karolinska Institute in Stockholm.

For the study, researchers examined data on singleton births in Sweden from 2007 to 2014.

Women with history of severe obesity

Researchers examined data on more than 33,000 women with a history of severe obesity who gave birth.

In the surgery group, women started out with an average body mass index (BMI) of 43.5 and an average weight of 122 kilograms. After surgery, their average body weight was 82 kilograms prior to conception.

Before gastric bypass surgery, 9.7% of the women in that group had been taking prescription diabetes drugs to lower their blood sugar. But in the six months prior to conception, only 1.5% of them took diabetes drugs.

Increased risk of birth defects

Both obesity and poorly controlled blood sugar are associated with increased risk of birth defects in a dose-dependent manner,” Neovius said by email. “This means that the more obese you are, the greater the risk of birth defects, and the worse blood glucose control, the greater the risk of birth defects.”

One limitation of the study is that it only included data on live births, not miscarriages or pregnancy terminations or stillbirths. It’s possible the results might underestimate birth defects by excluding these pregnancies, at least some of which might have ended because babies had birth defects too severe to survive.

All of the weight-loss surgeries were procedures known as Roux-en-Y gastric bypass.

Obesity can contribute to infertility

Obesity can contribute to infertility, noted Dr. Brian Smith, co-director of the minimally invasive surgery fellowship at UC Irvine Health and chief of surgery at the VA Long Beach Healthcare System.

Many women elect to undergo bariatric surgery in order to facilitate pregnancy and/or treat polycystic ovary syndrome, a major contributor to infertility,” Smith, who wasn’t involved in the study, said by email.