Weight loss surgery may reduce some of the pregnancy risks associated with obesity, but it could also put women and their infants at greater risk for other problems, according to a new study in Sweden.

Compared to obese women who didn't have so-called bariatric surgery, women who did were less likely to develop diabetes during pregnancy and less likely to deliver overly large babies, researchers report in The New England Journal of Medicine.

However, the women who had surgery were more likely to have shorter pregnancies and to deliver smaller-than-normal babies. There was also a possible link to increased risk of stillbirth and neonatal death among the babies of women who had the surgery.

Bariatric surgery "has both positive and negative influences on the risk of complications during a subsequent pregnancy,” said Kari Johansson, the study’s lead author from the Karolinska Institute in Stockholm.

Johansson cautioned in her email to Reuters Health that the study shows ties between weight-loss surgery and pregnancy outcomes, but doesn’t prove cause and effect.

Weight-loss surgery reduces the size of the stomach, and in some cases bypasses part of the digestive tract.

While it’s known that women who are overweight or obese have more pregnancy complications, the researchers say there is little information about pregnancy after weight-loss surgery.

Their new study involved nearly 3,000 Swedish pregnancies between 2006 and 2011 - including 596 in women who had weight-loss surgery within the previous five years, and another 2356 in women whose pre-pregnancy weight was similar to what the weight-loss surgery patients had weighed before their surgery.

Overall, there was no difference between the two groups in the risk of going into labor early. Also, there was no difference in the risk for birth defects.

Only about 2 percent of women in the surgery group developed diabetes during pregnancy, compared to about 7 percent in the non-surgery group. Also, about 9 percent of women in the surgery group delivered overly large babies, compared to about 22 percent of the non-surgery group.

But about 16 percent of women in the surgery group gave birth to smaller-than-normal babies, compared to about 8 percent of women in the non-surgery group. Also, the women who had surgery had pregnancies lasting 273 days, compared to about 278 days among the women who didn’t have surgery. Both of those durations are within the normal range, however.

The researchers also found that about 2 percent of pregnancies after weight-loss surgery ended in stillbirths or the death of infants within the first month of life, compared to about 1 percent of pregnancies in women who didn’t have surgery.

The researchers can’t say whether the increased risk of stillbirth or infant death was simply by chance, however.

“We therefore do not have a large enough sample to give any clear answer about this,” Johansson said. “This outcome needs to be studied further in even larger samples of patients.”

In an editorial, Dr. Aaron Caughey of Oregon Health and Science University in Portland wrote that other factors could influence complication rates, include complications during previous pregnancies.

Dr. Loralei Thornburg, who wasn’t involved in the new study, also pointed out that 98 percent of the women who had weight-loss surgery had the procedure that bypasses part of the small intestine.

“I think we have to interpret the findings with caution for different types of surgeries,” said Thornburg, who is a high-risk pregnancy expert at the University of Rochester Medical Center in New York.

She also said the study did not focus on many outcomes that may be better for mothers after weight-loss surgery, including their risk of death.

Thornburg, who said her center sees many women after weight-loss surgery, said women should always have discussions with their doctors before becoming pregnant – procedure or not.

She said women who had the surgery may need additional counseling about nutrition, care around remaining excess weight and possibly more monitoring. Additionally, they should talk about fertility before their surgery, she said.

“You should absolutely talk to your doctor,” Thornburg said. “Everyone should strive to be at a healthy weight and in a healthy lifestyle before they enter pregnancy.”