Women should remain active during pregnancy
Mothers-to-be who are obese increase their odds of having a baby with abnormalities including spina bifida, researchers say.
Their analysis of data from 39 studies found obese women were more than twice as likely to have a baby with spina bifida or another neural tube defect.
The risk of heart defects and cleft lip was also raised, the Journal of the American Medical Association says.
Up to a fifth of pregnant mothers in the UK are classed as obese.
Leading clinicians said the government should be focussing anti-obesity campaigns on women of child-bearing age.
It is well known that women who are obese are more likely to have difficulty conceiving and once they are pregnant, overweight and obese women and their babies are at a greater risk of a range of health problems.
However, this is the first time that so many studies have been combined to build a more accurate picture of the risks to the unborn child, according to lead researcher Dr Judith Rankin of Newcastle University.
"Given that we are seeing an increase in the number of people who are overweight or obese, then we may see an increase in the number of babies born with abnormalities," she said.
However, she stressed these abnormalities were uncommon.
"Spina bifida only occurs in approximately one in every 2,000 births, so the risk, even among obese women, remains very low.
"Women who are thinking about trying for a baby need to check their own weight first and then think about seeking help if they are overweight.
"While you are pregnant it's not the time to start a weight loss diet but it is more important to eat sensibly and healthily," she said.
The Royal College of Midwives echoed this advice.
Dr Rankin's team will now continue the work to examine why there is a link between a mother's weight and abnormalities in the baby. They say it could be related to nutritional deficiencies or undetected diabetes in the mother.
Both maternal diabetes and a lack of folic acid are established risk factors for birth abnormalities, particularly neural tube defects which affect the brain and spinal cord.
Andrew Russell, chief executive of the Association for Spinal Bifida and Hydrocephalus, said: "Folic acid does seem to be involved. It is very important for women planning a pregnancy or likely to fall pregnant to take a folic acid supplement."
Experts from the Royal Society of Medicine believe the government should be funding campaigns to target women of child-bearing age.
BODY MASS INDEX
Calculated by dividing weight in kilograms by height in metres squared
Normal: 18.5 - 24.9
Overweight: 25 - 29.9
Obese: Above 30
The RSM's president of the Obstetrics & Gynaecology section, Professor Philip Steer, said: "We've had warnings on cigarette packets telling us that smoking can harm the unborn baby as well as the mother. But there seems to be little effort made to warn mothers about the risk obesity poses to mother and child."
A Department of Health spokeswoman said: "Our advice is that all women should aim to embark on pregnancy as fit and as healthy as possible. To this end, we have introduced a range of initiatives to help people stop smoking, including free NHS services, and eat healthy."
By 2015, it is expected there will be 2.3 billion overweight and more than 700 million obese adults worldwide.
Obese is considered as a Body Mass Index (BMI) over 30, while overweight is classed as a BMI over 25. BMI is calculated by dividing your weight in kilograms by your height in metres squared.