MODEL RELEASED. Asthmatic pregnant woman. Woman using an inhaler to control an attack of asthma. Most of the inhaler drugs used to control asthma are safe for pregnant women and their unborn children. The inhaler drugs work by dilating the bronchial airways in the lungs that have contracted during an asthma attack. The use of an inhaler allows air to pass to the lungs. Any drop in blood oxygen levels can affect the unborn child, so controlling asthma attacks is essential. A full-term pregnancy lasts 37-42 weeks. Near the end of a pregnancy, large doses of some inhaler drugs, to treat asthma emergencies, can slow or prevent birth.