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Women who smoke while pregnant should be aware that they are increasing the chance their baby will be born malformed, say experts.

The risk for having a baby with missing or deformed limbs or a cleft lip is over 25% higher for smokers, data show.

Along with higher risks of miscarriage and low birth weight, it is another good reason to encourage women to quit, say University College London doctors.

In England and Wales 17% of women smoke during pregnancy.

And among under 20s the figure is 45%.

Although most will go on to have a healthy baby, smoking can cause considerable damage to the unborn child.

Missing limbs

Researchers now estimate that each year in England and Wales several hundred babies are born with a physical defect directly caused by their mother's smoking.

Every year in England and Wales around 3,700 babies in total are born with such a condition.

The experts base their calculations on 172 research papers published over the last 50 years, which looked at maternal smoking and birth defects.

The findings, from 174,000 cases of malformation and 11.7 million healthy births, revealed that smoking increased the risk of many abnormalities.

The chance of a baby being born with missing or deformed limbs is 26% higher, and cleft lip or palate is 28% more likely.

Similarly, the risk of clubfoot 28% greater, and gastrointestinal defects 27% more. Skull defects are 33% more likely, and eye defects 25% more common. The greatest increase in risk - of 50% - was for a condition called gastroschisis, where parts of the stomach or intestines protrude through the skin.

Professor Allan Hackshaw, who led the research, suspects many women who smoke while pregnant do not know about these risks.

"There's still this idea among some women that if you smoke the baby will be small and that will make it easier when it comes to the delivery.

"But what is not appreciated is that smoking during pregnancy increases the risk of defects in the child that are life-long."

He said very few public health educational policies mention birth defects when referring to smoking and those that do are not very specific - this is largely because of past uncertainty over which ones are directly linked.

"Now we have this evidence, advice should be more explicit about the kinds of serious defects such as deformed limbs, and facial and gastrointestinal malformations that babies of mothers who smoke during pregnancy could suffer from," he said.

Of the 700,000 babies born each year in England and Wales, around 120,000 babies are born to mums who smoke.

Amanda Sandford of Action on Smoking and Health said: "This study shows some of the worst outcomes of smoking during pregnancy. Pregnant smokers will be shocked to learn that their nicotine habit could cause eye or limb deformities in their baby.

"There is clearly a need to raise awareness of these risks among girls and to ensure pregnant women are given all the support they need to help them quit smoking and to stay stopped after the birth."

Basky Thilaganathan of the Royal College of Obstetricians and Gynaecologists said women who struggled to quit should at least cut down on how much they smoke.

Professor Hackshaw said the risk was likely dose-related - meaning the more a woman smokes, the bigger the risk to her unborn child.