Pregnant women who refuse to slow down in the gym are warned that they risk having smaller babies, reports the Evening Standard.

New research suggests working out vigorously while pregnant can seriously damage foetal growth and that babies of women who continue with activities such as weight-training are at significant risk of being affected.

Childbirth charities say modern mothers, under intense pressure not to lose their figures, take unnecessary risks to keep fit. The findings follow an American study, which examined the effect of exercise on the development of unborn babies.

Volunteer mothers-to-be were divided into groups and given different exercise programmes, which they followed from the eighth week of pregnancy until the birth. Their babies were then weighed and measured.

The babies of those assigned to a high volume of exercise in mid-to-late pregnancy were significantly lighter and thinner than those born to women who eased off their fitness programme.

Professor James Clapp, who led the research at Case Western Reserve University in Ohio, said: “These findings indicate that a high volume of moderate intensity, weight-bearing exercise in mid and late pregnancy reduces foetal growth. A reduction in exercise enhances growth, with a proportionately greater increase in fat mass than in lean body mass.”

Previous research has suggested that regular vigorous exercise such as aerobics in the first six months of pregnancy has significant positive health effects. Other studies have indicated that women who exercise during pregnancy lose weight more quickly after the birth and enjoy improved mood and sleep patterns. Details: www.cwru.edu