They could also be putting them at risk of developing behavioural problems, eating disorders and mental health conditions such as schizophrenia.
Doctors have long known that obese pregnant women are more likely to suffer blood clots, but the long-term effects of a mother's overeating on her child's health are not well understood.
Now a review of existing research has found evidence that obese women or those who put on excessive amounts of weight when pregnant could be harming their children's development.
Scientists from McMaster University in Ontario, Canada, looked at a dozen studies. They found that research in the US showed children of obese mothers tended to have IQ scores five points lower than the results of those whose mothers were a normal weight.
Swedish studies showed children were more likely to have attention deficit hyperactivity disorder if their mothers were obese.
And work on Australian teenagers suggested a child's chance of having an eating disorder increased by 11 per cent for each point their mother's body mass index increased during pregnancy.
The researchers, whose findings are published in Obesity Review, believe hormonal and chemical changes may explain the results.
What do you think? More stress and guilt for pregnant women trying to do their best?
Or a wake-up call for overweight mums?