Smoking during pregnancy damages a baby's airways even before birth, research has shown.
Smoking during pregnancy is not advised
Experts found babies whose mothers smoked during pregnancy were born with smaller airways - making them more vulnerable to breathing problems.
Smoking has been linked to various health problems in babies, including prematurity and low birth weight.
The study was carried out by the Institute of Child Health, and Great Ormond Street and Homerton Hospitals.
The team, led by Professor Janet Stocks and Professor Carol Dezateux, found airflow through the breathing tubes was on average 20% lower in babies born to smoking mothers.
They found the effect appeared to remain with the child during the first 18 months of life.
The researchers measured the airway function of premature babies before they left hospital.
This enabled them to show that the changes in the baby's airways were present not just in the last weeks of pregnancy - but at least two months before the baby was due to be born.
The team also found that babies born to mothers who smoke and those with reduced lung function shortly after birth were much more likely to suffer from wheezing illnesses in the first few years of life.
Smoking in pregnancy
Mothers who smoke are at increased risk of having a stillbirth, miscarriage or premature infant
Smoking during pregnancy is associated with a 3-4 fold increased risk of cot death
Babies born to mothers who smoke are significantly lighter and shorter than those born to non smokers
Children who are exposed to tobacco smoke before birth or in the home are far more likely to suffer from respiratory illnesses
Professor Stocks said: "This research adds to a large body of evidence which highlights the health consequences for babies of maternal smoking during pregnancy.
"Every effort must be made to discourage women from starting to smoke and to help those who do smoke to stop, whether during pregnancy or after delivery.
"This would not only reduce the incidence and severity of respiratory illnesses in infants and children, but would reduce inequalities in health and improve the health of young children in general since exposure to smoking at home is by far the most important source for young children."
Figures show approximately a third of women in the UK smoke, and very few of these give up while pregnant.