Light exercise during pregnancy may improve the future health of a child by controlling weight in the womb, New Zealand and US researchers say.
Overweight or obese mums are more likely to have larger babies which could be at higher risk of health problems later in life.
A study of 84 first-time mothers found exercise was associated with slightly lighter babies.
UK guidelines recommend regular light exercise for pregnant women.
The rising weight of the UK population over the years has led to a rise in the number of overweight mothers.
There is increasing evidence that the future metabolism of a child may be influenced by its environment in the womb, and that babies who are relatively heavy for their length may be more likely to be obese in future years.
Official guidance in the UK tells doctors to encourage women not to overeat during pregnancy, and to, wherever possible, take light exercise on a regular basis.
The joint study between the University of Auckland and Northern Arizona University recruited pregnant women, half of whom were asked to use exercise bikes for five 40 minute sessions each week.
They were asked to maintain the programme until at least the 36th week of pregnancy.
On average, the exercising women had babies who were no shorter than their non-exercising counterparts, but who were 0.32 lb (143 grams) lighter on average.
This suggested that the regime did not stunt growth in the womb, but reduced the amount of extra fat laid down by the babies.
In addition, the exercise did not appear to interfere with the natural changes in the mother's response to the hormone insulin, a necessary mechanism in pregnancy to make sure the foetus is properly nourished.
The findings are published in the Journal of Clinical Endocrinology & Metabolism.
Dr Paul Hofman, who led the study, said: "Given that large birth size is associated with increased risk of obesity, a modest reduction in birth weight may have long-term health benefits for offspring by lowering this risk later in life."
Dr Anne Dornhorst, who specialises in research into the metabolism of pregnant women, said that it was becoming clear that exercise during pregnancy could aid the health of mother and child.