The British scientists said the quitting rate was about the same as for people who use nicotine replacement.

"Our findings suggest that a physical activity intervention is feasible and acceptable as an aid to smoking cessation during pregnancy," Michael Ussher and colleagues from St. George's University of London, wrote in the journal 'BMC Health'.

Nicotine -- one of the most addictive substances known -- can lead to lower birth weight, higher infant mortality and is linked to learning difficulties and health problems in childhood.

An estimated 17 per cent of British women and 20 per cent of women in the United States say they smoke during pregnancy, the researchers said.

Nicotine patches are one way to help smokers give up, but there are worries they may harm the fetus, leaving exercise as a healthy alternative for pregnant women, the researchers said.

The two pilot studies included women over 18 who smoked at least one cigarette a day and were 12 to 20 weeks into their pregnancy.

In one trial, women exercised under supervision once a week for six weeks while in the other they worked out twice a week for the same length of time and then once weekly for three weeks. All received counselling and help to stop smoking.

The women exercised at a moderate pace and the main activity was walking, the researchers said. A quarter of the 32 women gave up smoking before giving birth, the studies found.

The researchers, who are now conducting a larger trial of more than 850 women, did not say why exercise appears to make a difference but previous research has linked physical activity to reduced cravings during pregnancy.

"Regular exercise is ideal for any pregnant women who smoke as it's obviously safe and the benefits are enormous," Ussher said in a statement.