Experts have urged overweight women trying to have a baby to slim down before sex.
The call comes after new research showed a rising risk of serious birth defects as the body mass index (BMI) of mothers increased.
Women of reproductive age should be encouraged to "adopt a healthy lifestyle and to obtain a normal body weight before conception", the authors said.
It was already known that being obese heightens the chances of a woman giving birth to a child with malformations that can affect physical appearance or the functioning of nerves and organs.
But the new study revealed a sliding scale of risk that went up as weight increased.
Researchers analysed data on more than 1.2 million live singleton births in Sweden, recorded between 2001 and 2014.
Among normal-weight mothers with a BMI of 18.5 to 24, 3.4 per cent had children with birth defects.
Overweight mothers with a BMI of 25 to 29 had a birth defect risk of 3.5 per cent.
The rate of birth defects went up to 3.8 per cent for obese women with a BMI of 30 to 34, and rose to 4.2 per cent and 4.7 per cent for higher categories of obesity.
BMI is a measurement obtained by dividing weight in kilograms by height in centimetres squared.
The scientists were led by Martina Persson, of Sweden's Karolinska Institute, and the findings were published in the British Medical Journal.
