Risk of birth defects increases in correlation with the mother's weight, major study shows

Overweight women should lose weight before starting a family, experts have warned, as new evidence reveals the risk of birth defects increases with the mother's weight.

A major study which tracked 1.2million births revealed a sliding scale of risk to the baby that increases as weight goes up.

'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 scientists wrote in the British Medical Journal.

Experts already knew obesity heightens the chances of a woman giving birth to a child with health problems.

But the new study reveals the exact degree of that risk for the first time - and shows that the chance of problems starts from being only slightly over what is considered a 'healthy' weight. 

How was the study carried out? 

Researchers from Sweden's Karolinska Institute analysed data on births in that country recorded between 2001 and 2014.

They found being overweight increased the risk of birth defects including heart problems, defects of the limbs, genital problems, urinary issues, digestive problems and malformed nervous systems.

Among 'normal' weight mothers, classed as having a 'body mass index' or 'BMI score' of 18.5 to 24, a total of 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.

Obesity increased the risk 

The researchers, led by paediatrician Martina Persson, wrote: '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 [weight] in those with a BMI above the normal range.'

It comes after a global study earlier this week revealed 67 per cent of adult men in Britain and 57 per cent of adult women are overweight.

Of these, 24 per cent of British adults - 12 million people – are considered obese, a vast increase since 1980, when only 16 per cent were in this category.

'The importance of maintaining a healthy weight' 

Tam Fry, of the National Obesity Forum, said last night: 'It is astonishing that in a highly developed country like the UK, preconception education is so woeful.

'It should be therefore a fact of life that girls, from the first years of secondary school, get advice on the importance of maintaining a healthy weight.

'This would be good not only for their own self esteem but also for the responsibility they bear if and when they decide to have a child.'

Jacque Gerrard, director for England at the Royal College of Midwives, added: 'Adult and childhood obesity is a major problem in the UK which is known to increase the risk of developing other associated disease such as diabetes, hypertension and cardiovascular disease.

'We need to see more effort and resources put into reducing obesity generally in the population, and in the services available to women who are planning a family or already pregnant.' 

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