Babies of mothers who smoke are more likely to be born prematurely (before a full nine months of pregnancy). Babies who are born too soon may die, and if they survive, they may have to stay in the hospital for a long time. Some of them have lasting health problems as a result of their early births. It has been estimated that smoking is responsible for 15% of all premature births. (1) Smoking during any stage of pregnancy, not just the last few months, increases the risk of premature birth.
Sources 1. Andres RL, Day MC. Perinatal complications associated with maternal tobacco use. Seminars in Neonatology 2000;5:231-241.