Babies of women who do not gain enough weight are as likely to be obese as those whose mothers gain too much, they said.
And surprisingly, the risk is higher for women who are a normal weight at the start of their pregnancy.
With obesity taking up to nine years of a person’s life and sowing the seeds of a host of health problems, British experts said it is imperative that women are given clear advice on how much weight to gain during pregnancy.
The U.S. researchers tracked the health of more than 4,000 pregnant women and their children.
Overall, some 20.4 per cent of boys and girls born to women who put more weight than advised in pregnancy were overweight or obese between the ages of two and five.
This is similar to the 19.5 per cent of children born to women who gained less than the recommended amount of weight – and more than the 14.5 per cent among women who stuck to the guidelines.
The figures for women who went into pregnancy at a healthy weight were particularly striking, the American Journal of Obstetrics and Gynecology reports.
A slim woman who ate too much in pregnancy was 80 per cent more likely to have a fat child than one who ate the right amount.
And one who ate too little was 63 per cent more likely to have a child who was overweight or obese.
The finding that the children of slim women were particularly affected suggests that genes cannot be completely blamed for the effect.
Instead, it is thought that conditions in the womb programme the unborn child’s metabolism for years to come.
For instance, there could be long-lasting consequences for the unborn child’s appetite control or storage of fat.
Researcher Dr Monique Hedderson, of the Kaiser Permanante Division of Research in Oakland California, said: ‘Gaining either too little or too much weight in pregnancy may permanently affect mechanisms that manage energy balance and metabolism in the offspring, such as appetite control and energy expenditure.
‘This could potentially have long-term effects on the child’s subsequent growth and weight.’
Dr Geeta Nargund, of the Create fertility clinic in central London, said the study adds to increasing evidence that conditions in the womb can have long-lasting effects on the child’s health.
She added that it emphasises the importance of providing advice to women before and during pregnancy.
Tam Fry, of the Child Growth Foundation, said it is imperative that the UK follows the US’s lead in issuing clear advice on weight gain.
The NHS simply advises that weight gain in pregnancy ‘varies greatly’, with most women putting on between 22lb to 26lb.
In contrast, the U.S. advises how much women of different weights should gain.
For instance, a woman who goes into pregnancy at a normal, healthy weight, should put on 11 to 20lb.