Exercising may be a safe way for pregnant women to quit smoking, early research suggests.

Nicotine can lead to lower birth weights and higher infant mortality and is linked to learning difficulties, problem behaviour and asthma in childhood.

In Tuesday's online issue of the journal BMC Public Health, British researchers said they found 25 per cent of women who exercised regularly while expecting were able to kick the habit, a success rate comparable to using nicotine replacement patches and gums to quit. 

'Regular exercise is ideal for any pregnant women who smoke as it's obviously safe and the benefits are enormous.' — Study author Michael Ussher

Nicotine replacements haven't been well tested in pregnant women and may harm the fetus, the researchers said.

"These results are very encouraging and we are now conducting a randomized controlled trial with 850 women," said study author Michael Ussher of the division of community health sciences at St. George's University in London.

"Regular exercise is ideal for any pregnant women who smoke as it's obviously safe and the benefits are enormous."

The two pilot studies included 32 women over 18 who smoked at least one cigarette a day and were 12 to 20 weeks into their pregnancy.

The first group exercised once a week for six weeks, while a second group exercised twice a week for six weeks, then once a week for three weeks.

Researchers supervised the exercise, which included walking on a treadmill, and encouraged the volunteers to exercise more often if they were able to do so.

"Women reported that the intervention helped weight management, reduced cigarette cravings and increased confidence for quitting," the study's authors wrote.

"Our findings suggest that a physical activity intervention is feasible and acceptable as an aid to smoking cessation during pregnancy."

The researchers checked that women were abstaining from smoking by measuring expired carbon monoxide for up to eight months gestation.

Future studies will look at whether exercise also prevents women from relapsing into the habit after they give birth.