One in every eight mums-to-be is smoking during pregnancy, it has emerged on National No Smoking Day today.

And some women continue to light up during pregnancy because they think the habit will lead to smaller babies and easier births.

Oxfordshire Primary Care Trust believes the increase of smoking mums has been caused by the county's growing Eastern European population, and as a result, health advisers are working closely with the Polish community.

From April to September 2006, there were 3,845 births, with 487 mothers smokers at the time of delivery - 11.2 per cent.

For the same period in 2007, there were 3,801 births, of which 426 mothers were smokers at time of delivery - 12.5 per cent.

Jane Krebs, public health midwife at the John Radcliffe Hospital, said: "I think smoking is the norm in their society, so while we perceive someone who smokes 20 a day as a heavy smoker, they may not.

"The support is there for them and we need to get information to them in their own language."

Ms Krebs said some mums-to-be would not give up because they falsely believed it would help them have a pain-free delivery.

She said: "Women know smoking leads to much smaller babies, but in their naivety they think that's a benefit.

"But it just doesn't work out that way. One woman can deliver an 8lb baby with ease, while another will struggle with a six pounder."

Mums attending a teenage pregnancy support group said they had heard that smoking while pregnant caused smaller babies.

Michaela Hayward, 19, from Iffley Road, Oxford, smoked while pregnant with her daughter.

She said: "I was only smoking about five fags a day when I was pregnant but before I was smoking about 10 or 15.

"I cut down because I read babies came out smaller but not as healthy if you keep smoking."

Family support worker Claire Johnson, 40, from Blay Close, Blackbird Leys, said she smoked while pregnant with her three children.

She said: "There is not enough awareness about it."

Mrs Krebs works closely with PCT smoking advice workers, as well as the county's community midwives, to encourage pregnant women to quit their habit.

Smoking leads to development problems among babies, as well as respiratory conditions like asthma, and has been linked to Sudden Infant Death Syndrome.

Teenage pregnancy support worker Carole Partlett said about half the young mums she worked with smoked, and none gave up during pregnancy. She added: "I can't say I've had many who actually tell me it's because they want smaller babies, but one or two do."

A spokesperson for charity Advice on Smoking and Health said: "It is very sad indeed to hear that the myth of an easier birth is still in circulation."