Research has found women who stop smoking in the first 15 weeks of pregnancy reduce their risk of having premature or small babies to that of a nonsmoker.
According to a study in the British Medical Journal, scientists in New Zealand found that the risk of two potential problems caused by the habit are reduced when expectant mothers stop smoking early in their pregnancy.
Previous research has confirmed that smoking in pregnancy increases the risk of complications such as miscarriage and still birth.
In the survey of more than 2,500 pregnant women at the 15-week stage, scientists at the University of Auckland discovered for the first time the effect of giving up smoking for pregnant women.
The women were divided into three groups, 80% nonsmokers, 10% current smokers and 10% stopped smokers (who gave up before reaching week 15).
The results show there was no difference in the rates of premature birth and baby size between stopped smokers and nonsmokers, but the rates were much higher for current smokers.
Dr Lesley McCowan, who led the study, said: "In women who stopped smoking before 15 weeks' gestation, rates of spontaneous preterm birth and small-for-gestational-age infants did not differ from those in nonsmokers, indicating that these severe adverse effects of smoking may be reversible."