(CNN) - A new study advises women to adopt a healthy lifestyle and be at a normal body weight before conception.
That's because the risk of major birth defects increases in step with the mother's extra weight.
The study was published in the Medical Journal BMJ.
Researchers looked at more than a million births in Sweden between 2001 and 2014.
For mothers of a normal body mass index, the study found a 3.4 percent risk of a major birth defect.
For overweight mothers, there was a slight uptick to 3.5 percent.
But for obese mothers, the risk of a birth defect grew noticeably - up to a 4.7 percent rate for women with a BMI of 40 or greater.
A medical adviser for the March of Dimes who looked at the study says the findings are in line with past research and stresses that being at a healthy weight has benefits for both mother and infant.
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