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Women who smoke beyond 32 weeks of pregnancy are almost 50 per cent more likely than non-smokers to have a baby with breathing problems. A study of more than 14,000 mothers in Bristol found that if a woman smokes at 18-20 weeks of pregnancy the baby is 40 per cent more likely to have a baby that wheezes in its first six months of life, but by 32-34 weeks the likelihood increased to 47 per cent .

Dr John Henderson, the consultant senior lecturer at Bristol University's Institute of Child Health, said: "There is a clear message that the risk increases progressively through the first trimester and the second trimester to the end of pregnancy."

Smoking during pregnancy has also been shown to increase the risk of miscarriage and stillbirth and babies of smokers are likely to be underweight. They are twice as likely to be born premature and have more chance of asthma, glue ear and chest infections.