A woman's risk of having a baby with a low birth weight is higher if she starts smoking in her teens, a new study suggests. That's because women who start smoking as teens are far more likely to smoke during pregnancy, which increases the risk of having a smaller baby. In turn, babies who have low birth weights are more likely to have physical and mental problems later in life. Researchers analyzed data from a long-term U.S. government study of nearly 21,000 women who were (..)