听力原文： Through a large amount of research, many American doctors have drawn the conclusion that mothers who smoke cigarettes before their babies are born may slow the growth of their babies' lungs. They say reduced lung growth could cause the babies to suffer breathing problems and lung disease in their later life. John Hailor and William Liburdy, doctors in Boston, Massa chusctts studied about 1, 100 children to look into the effect that mothers' smoking had on their children. The mothers of some of the children smoked, the other mothers did not. The report indicated that the growth of babies' lungs was indeed affected by second-hand smoking. Doctors found that the lungs of the children whose mothers smoked were eight percent less developed than the lungs of the children whose mothers did not smoke, and that the children whose mothers smoked developed twenty percent more colds and breathing diseases than other children in their later life. Another recent study found that children had a greater chance of developing lung cancer if their mothers smoked. The study also showed that the danger of lung cancer increased only for sons and not for daughters, and that the father's smoking did not affect a child's chance of developing lung cancer. Why it goes like that is still not known. (30)