Women who have undergone bariatric surgery for weight loss may run a higher risk of giving birth to premature babies, a new study suggests.

The findings, published in the British Medical Journal, revealed how the weight-loss surgery affects pregnancy. Swedish researchers observed babies born to women who have undergone bariatric surgery and found that they had a greater chance of being born smaller for their gestational age, Fox News reports.

"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," Dr. Olof Stephansson, obstetrician and associate professor at the Clinical Epidemiology Unit at Karolinska Institutet, said in a statement which suggests that the pregnancies should be considered high risk and monitored carefully.

The study observed more than 2,500 babies born between 1992 and 2009 to women who have had weight-loss surgery and compared them to 12,500 babies born to mothers who did not have surgery. Each pregnancy was matched individually to factor in the mother’s body-mass index, age, educational background and smoking habits.

Researchers found the babies born to mothers who had bariatric surgery had lower weights – 5.2 percent of the babies were considered small compared to 3 percent of babies born to mothers who didn’t have bariatric surgery. Additionally, 9.7 percent of babies were born early compared to 6.1 percent of babies whose mothers had no surgical history.

"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," Stephansson said.

While the surgery may be linked to a higher risk of premature babies, previous studies have drawn contrary conclusions. For instance, a JAMA study in 2008 saw a positive relation between bariatric surgery and pregnancy – but that study involved just 77 patients.

"Given this relative dearth of data, any new data are a welcome increase in knowledge and can be expected to improve outcome estimates of the true effect," Paul Shekelle, MD, PhD, MPH, senior author of the 2008 JAMA, study told MedPage Today about the latest results. "With these new data, the conclusion would certainly be that having received bariatric surgery puts a woman in a higher risk group, for sure, but whether that woman is at higher or lower risk compared to her pre-bariatric surgery weight and health status is still an open question."