Women who are obese when they conceive are more likely to have a baby with serious birth defects, a major study has found.
The research revealed a sliding scale of risk for health problems including congenital heart defects, anomalies of the digestive system and malformations of genital organs or limbs.
The study is the first to show that babies of women who are overweight, but not clinically obese, are also slightly more likely to have health problems in the first year of life.
The study, which used data from more than 1.2 million live births in Sweden between 2001 and 2014, provides some of the most compelling evidence so far of link between obesity in pregnancy and birth defects. While the additional risk was small for women who were just above the healthy weight range, the progressive increase in risk made the researchers more confident that the link was causal.
Martin Neovius, an epidemiologist at the Karolinska Institute and the study’s senior author, said: “In terms of risk, it is better to be normal weight than overweight and much better to be normal weight than obese.”
Writing in the British Medical Journal, the researchers urged women who are planning pregnancy to aim to get their weight within the healthy range before conception.
“Efforts should be made to encourage women of reproductive age to adopt a healthy lifestyle and to obtain a normal body weight before conception,” the authors wrote.
Neovius added that the sliding scale of risk suggests that even decreasing from severe obesity to milder obesity would have a benefit. “It’s very hard for someone of a BMI of 42 to become normal weight,” he said.
The study found that a total of 43,550 babies (3.5% of all births) had major congenital malformations. Compared with mothers in the healthy weight range (3.4% risk of defects), the risk was 3.5% for overweight mothers, 3.8 for obese mothers, and rose to 4.2% and 4.7% for higher categories of obesity.
The categories were based on body mass index (BMI), with a healthy weight defined as a BMI between 18.5 and 24, overweight being 25 to 29, and obese being 30 or over.
The researchers took into account a range of potential confounding factors, and their findings are strongly suggestive – although not absolute proof –of a causal link. Scientists are still trying to understand how maternal weight impacts the developing foetus.
Altered hormone levels, higher levels of inflammation and less efficient delivery of nutrients via the placenta could all play a role, the paper suggests. Poor blood sugar control was cited as another potential risk factor, although the link remained when those diagnosed with diabetes were excluded.
Another possible contributor is that overweight women have been shown previously to be less likely to take folic acid supplements, which are know to protect against certain birth defects. The latest study was not able to control for this possibility.
Neovius said that the risk of birth defects was just one of many of additional risk factors faced in pregnancy by women who are obese.
“Severe obesity confers excess risk for so many other negative outcomes for pregnancy: pre-eclampsia, diabetes, still-birth,” he said. “It’s really not a good place to be.”
He acknowledged though that losing weight, especially for those who are severely obese, poses a significant challenge and that few interventions have been shown to be consistently successful.
Globally, obesity is viewed as a growing health problem, with the number of women aged 18 years and older with a BMI greater than 35 having doubled from 50 million to 100 million between 2000 and 2010.
