Bariatric Surgery Affects Fetal Growth, Linked to Preterm Birth


Babies born to women who have undergone bariatric surgery for weight loss were more likely to be premature and to be small for gestational age.

Babies born to women who have undergone bariatric weight-loss surgery were more likely to be premature and to be small for gestational age, according to a large Swedish study.

These findings are in line with the results of a similar study conducted in Denmark.

Researchers from the Karolinska Institute in Solna, Sweden, evaluated 1.74 million singleton births identified in the Swedish medical birth register between 1992 and 2009 to look for the association between bariatric surgery and perinatal outcomes. Ultimately, the study compared about 2500 babies born to women who had previously undergone bariatric surgery with 12,500 babies born to mothers who had not. The pregnancies were matched individually, so that the mothers' BMI, age, educational background, smoking habits, and previous births were comparable in both groups.

Preterm birth was identified in nearly 10% of deliveries from mothers who had a history of bariatric surgery, compared with 6.1% of babies born before 37 weeks to women who had not had the surgery.

Mothers who had undergone bariatric surgery as well as had a BMI of 30 or less in early pregnancy were at the highest risk of delivering their babies early, the researchers found. However, there was no increased risk observed in women who had bariatric surgery and an early pregnancy BMI of 35 or greater.

In addition, babies born to mothers with bariatric surgery history had lower birth weights, with 5.2% considered small for gestational age and at least two standard deviations below the normal as opposed to only 3% in the control group. Not only were the babies more likely to be small, but the chance of a women giving birth to a child who was large for gestational age was also reduced if the mother had a history of bariatric surgery. (Just 4.2% of babies born to mothers with bariatric surgery history were large, compared with 7.3% of the control group.)

“Pregnant women with a history of bariatric surgery should be regarded as a risk group and be counseled about the increased risk of preterm birth and intrauterine growth restriction compared with pregnant women with similar characteristics,” the authors concluded. They also suggested that additional studies are needed to understand the reasons and causes behind the increased risk.

"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," said study author Olof Stephansson, MD, obstetrician and associate professor at the Clinical Epidemiology Unit at Karolinska Institute in a statement. "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."

Pertinent Points:
- Babies born to women who have had bariatric surgery should be considered at risk for preterm birth and low gestational birth weight, according to a new population-based study conducted in Sweden.
- The risk of premature births for babies born to mothers with a history of the weight loss surgery was only found in women who also had an early pregnancy BMI of 30 or below. Those with a BMI of 35 or greater showed no increased risk.


Roos N, Neovius M, Cnattingius S, et al. Perinatal outcomes after bariatric surgery: nationwide population based matched cohort study. BMJ. 2013;347:f6460.

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