Gastric bypass surgery has given new lives to people who are severely obese. The procedure allows food to be rerouted past most of the stomach so less is digested, and some patients have experienced up to a 70 percent loss of excess body weight within two years. However, the surgery can have adverse side effects, such as vitamin and mineral deficiencies, as it reduces the body’s ability to absorb micronutrients.

A new study by researchers at University Hospital in Angers, France found another reason for concern: Women who have undergone gastric bypass may be more likely to have low birth weight babies.

Women who have had the procedure typically are told to wait 18 months after surgery to try to get pregnant. Even then, researchers found that the birth weights of infants born to gastric bypass mothers were, on average, 12 ounces lower than those born to other mothers. The results also found that 23 percent of these infants were small for their age.

The authors suggest that one cause of the low birth weight could be that some of the gastric bypass mothers were deficient in calcium and zinc, key nutrients during pregnancy. An analysis of the newborns showed that they, too, had lower-than-average levels of the same nutrients.

While the research showed some areas of concern about gastric bypass, the study’s author noted that obesity can also result in risky pregnancies and can lead to high birth weight and low blood sugar in newborns.

So women who choose to have the procedure before trying to get pregnant should remember that excess weight is not the only thing they may lose. Vital nutrients also could be diminished and moms-to-be should talk to their physician about appropriate nutritional supplements.