London, Sep 10 (IANS): Women who undergo gastric bypass surgery for weight loss before their pregnancy may increase the risk of giving birth to babies that are small in size or have lower average birth weights, a study has found.

Maternal obesity can lead to various health conditions for the newborn, such as high birth weight and low blood sugar as well as also cause birthing complications. Gastric bypass surgery can prevent these.

However, "our study has showed that gastric bypass could have other effects on newborns," said Maxime Gerard, lead researcher at the University Hospital of Angers in France.

Gastric bypass surgery is used to treat people who are severely obese (a body mass index greater than 40kg/m2). The procedure includes re-routed food past most of the stomach, meaning less is digested.

The surgery can lead to up to 70 per cent loss of excess body weight within two years, but is at the same time associated with an increased vulnerability to vitamin and mineral deficiencies, as it reduces the body's ability to absorb micronutrients.

Women treated with gastric bypass surgery are advised to wait 18 months after the procedure before trying to become pregnant in order to establish a stable, healthy weight.

They must also follow a daily multi-vitamin supplementation regime and receive regular clinical follow up before, during and after the pregnancy, the researcher said.

The findings showed that the birth-weight of babies born to gastric bypass mothers was on average 0.34 kg lower than average, and that 23 per cent of neonates were small for their gestational age.

Despite adequate supplementation, a proportion of gastric bypass mothers were deficient in key nutrients during pregnancy, such as calcium and zinc.

Analysis of the newborns showed that they also suffered lower than average levels of the same nutrients.

"These maternal nutrient deficiencies may be the reason for the same deficiencies and low birth weights seen in the newborns," Gerard added.

For the study, the team analysed 56 newborns born to gastric bypass mothers who had waited an average of 32 months between surgery and pregnancy, and compared results to 56 controls.

The team also analysed newborn birth weight in relation to the mothers' weight, and determined that birth weight was related to the variation in the mother's weight between the surgery and pregnancy, rather than her weight during pregnancy.

The results were presented at the 55th Annual European Society for Paediatric Endocrinology Meeting, in Paris recently.