US researchers found that the more obese a mother is when she becomes pregnant, the greater her risk of giving birth to a baby with a congenital heart defect.

You can read about the study, by investigators at the National Institutes of Health (NIH) and the New York state Department of Health, in the 7 April online issue of the American Journal of Clinical Nutrition.

In the US, nearly one third of pregnant women are obese (with around one fifth already obese from the start of their pregnancy), and previous studies show that the condition leads to many congenital complications, including neural tube defects, serious malformations of the spinal column.

However, as the authors explained in their background information, what is not so clear is how maternal obesity affects the risk of babies being born with heart defects, the most common of congenital malformations, affecting 8 in every 1,000 newborns in the US.

Congenital heart defects comprise a problems in the structure of the heart and range from minor to life threatening.

The researchers found that on average, being obese raised a woman's chance of giving birth to a baby with a heart defect by about 15 per cent, and the higher her Body Mass Index or BMI, the higher the risk.

They found the risk went up sharply at a BMI of 30 and progressed higher with increasing BMI.

BMI is a measure of obesity that takes the person's weight in kilos and divides it by the square of their height in metres. A BMI of 18.5 to 24.9 is classed as normal, 25 to 29.9 is overweight, 30 and over is obese.

The results showed that moderately obese women (BMI 30 to 39.9) had an 11 per cent higher risk, while morbidly obese women (BMI of 40 and over) had a 33 per cent higher risk of having a baby with congenital heart defects.

They also showed, that on average, women who were overweight (BMI 25 to 29.9) but not obese had no increased risk.

First author Dr James L Mills, of the Division of Epidemiology, Statistics and Prevention Research at the Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD), one of the NIH Institutes, commented to the media that:

'The trend is unmistakable: the more obese a woman is, the more likely she is to have had a child with a heart defect.'

For the study, Mills and colleagues analyzed data on 1.53 million births from 1993 to 2003 kept in the New York State Congenital Malformations Registry, which holds records of children born with birth defects in New York state, but not New York City. This allowed them to compare mothers of 7,392 children born with major heart defects against more than 56,000 mothers of babies without congenital defects.

They concluded that:
'Obese, but not overweight, women are at significantly increased risk of bearing children with a range of congenital heart defects, and the risk increases with increasing BMI.'

The researchers recommended that further studies should now be done to investigate whether weight reduction can help obese women reduce the risk of their baby being born with heart defects.

Because this study only looked at records of babies after they had been born, and didn't compare obese women who reduced weight in pregnancy against women who did not, it can't say for certain whether weight reduction reduces the risk or not.

However, the results were strong enough to make Dr Alan E. Guttmacher, acting director of the NICHD, say that:

'The current findings strongly suggest that by losing weight before they become pregnant, obese women may reduce the chances that their infants will be born with heart defects.'

Mills agreed:
'If a woman is obese, it makes sense for her to try to lose weight before becoming pregnant,' he urged.

'Not only will weight loss improve her own health and that of her infant, it is likely to have the added benefit of reducing the infant's risk for heart defects,' said Mills.

Other studies have shown that being obese while pregnant is linked to several types of complication, for both mothers and their babies.

According to the NIH, obesity in pregnancy increases the risk of high blood pressure, preeclampsia (a serious form of high blood pressure that occurs during pregnancy), gestational diabetes, and cesarean delivery.

Also, babies born to women who are obese in pregnancy are themselves at higher increased of becoming overweight and developing type 2 diabetes later in life.