A pregnancy normally lasts 40 weeks from the last menstrual period. A birth that occurs at less than 37 weeks is considered to be premature. Before 20 weeks, any birth that occurs is considered to be a miscarriage. It is rare for an infant born before 24 weeks to survive, and only after 30 weeks are the chances of survival considered to be good.
Premature labour occurs in about 7% of pregnancies. There is no apparent cause in over half the cases, but in others, high blood pressure, diabetes, two or more babies, more than six previous pregnancies, foetal abnormalities, polyhydramnios and abnormalities of the uterus may be responsible.
Premature labour may now be prevented or controlled in some cases by injections of drugs such as atobisan, ritodrine (Yutopar) or salbutamol (Ventolin, which is also used to treat asthma). Strict bed rest is the only other form of treatment.