Mothers-to-be are being urged to exercise during pregnancy in order to minimize their chances of having an obesity-prone baby.
New research revealed that babies are born smaller when the mother took part in fitness training on exercise cycles, compared to those who remained inactive during pregnancy.
Scientists said there was no evidence that babies suffered nutritionally or were shorter in length.
Dr Paul Hofman, from the University of Auckland in New Zealand, said: "Our findings show that regular aerobic exercise alters the maternal environment in some way that has an impact on nutrient stimulation of foetal growth, resulting in a reduction in offspring birth weight.
"Given that large birth size is associated with an increased risk of obesity, a modest reduction in birth weight may have long-term health benefits for offspring by lowering this risk in later life."
The trial involved 84 first-time mothers who were randomly assigned either to an exercise group or a non-exercise "control" group.
Exercise consisted of stationary cycling and involved a maximum of five 40 minute sessions per week.
The women taking exercise were asked to maintain the programme until at least the 36th week of pregnancy.
All the study participants were tested for their sensitivity to insulin, the essential hormone that regulates the body's use of sugar. One concern about exercise during pregnancy has been that it could prevent maternal insulin resistance.
Normally insulin resistance, which reduces the body's ability to respond to insulin, is harmful and can lead to Type 2 diabetes. But temporary insulin resistance during pregnancy is necessary to ensure enough nutrients reach the foetus.
In the study, the women who exercised had babies that were on average 143 grams lighter than those of women who did not exercise.
However moderate aerobic exercise had no impact on insulin resistance and had no effect on the body weight of women in late pregnancy.
"The physiological response to pregnancy appears to supersede the chronic improvements in insulin sensitivity previously described in response to exercise training in non-pregnant individuals," said Dr Hofman. "This may be an important finding for athletes who want to continue regular training during their pregnancy as it suggests that training will not have a major adverse impact on insulin resistance."
The findings were reported in the Journal of Clinical Endocrinology & Metabolism.