The birth of a baby before 37 weeks of pregnancy. In humans, a normal pregnancy lasts about 40 weeks. The risk of preterm birth may be increased by certain health problems in the mother, such as diabetes, heart disease, and kidney disease, or problems during pregnancy. Smoking cigarettes, being exposed to secondhand tobacco smoke, drinking alcohol, and taking certain drugs during pregnancy may also increase the risk of a preterm birth. Also called premature birth.