Experts have urged overweight women trying to have a baby to slim down before trying to conceive. 

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," said the authors.

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.

Poor diet in pregnancy can affect the future health of unborn children. We already advise women to aim for a healthy weight before pregnancy and to be active and eat healthily while pregnant.

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, a total of 3.4 percent had children with birth defects.

Overweight mothers with a BMI of 25 to 29 had a birth defect risk of 3.5 percent. The rate of birth defects went up to 3.8 percent for obese women with a BMI of 30 to 34, and rose to 4.2 percent and 4.7 percent for higher categories of obesity.

BMI is a measurement obtained by dividing weight in kilograms by height in centimetres squared. It takes account of different stature when assessing weight.

The scientists, led by Sweden's Karolinska Institute's Martina Persson, wrote in the British Medical Journal: "Risks of any major congenital malformation and several subgroups of organ specific malformations progressively increased with maternal overweight and increasing severity of obesity.

"For women who are planning pregnancy, efforts should be encouraged to reduce adiposity in those with a BMI above the normal range."

Head of nutrition science at Public Health England, Prof Louis Levy, said: "Poor diet in pregnancy can affect the future health of unborn children. We already advise women to aim for a healthy weight before pregnancy and to be active and eat healthily while pregnant."