Despite knowing the risks to their children's health, parents continue to expose their children to tobacco smoke prior to and after their birth. This study explores the factors influencing parent's behaviour in preventing the exposure of their (unborn) children to environmental tobacco smoke (ETS) and any changes to their smoking behaviour in the home during the first years of their children's lives. Whether or not they stopped smoking during pregnancy, the women did not protect themselves from breathing in other people's smoke. Yet once the baby was born, parents actively protected the baby from environmental tobacco, believing that the lungs of newborn babies were too immature to tolerate smoke. This protection lasted only for a matter of weeks for some babies, or stopped when they were 6-12 months old, linked to their parent's belief that older babies could tolerate or avoid smoke. These findings suggest that changes made to smoking during the first weeks of a baby's life are unlikely to be sustained, and key messages about the risks if ETS exposure need to be delivered repeatedly over the first 2 years of life and re-enforced as the child gets older.