An obstetrical complication in which the placenta is implanted into a lower than usual segment of the uterus, partially or completely obstructing the cervical opening to the vagina. It is the main cause of antepartum haemorrhage (vaginal bleeding) and if severe enough may lead to a cesearian section or preterm birth, especially if it remains unresolved beyond 20 weeks of pregnancy. In the US, placenta previa is reported to have an incidence of 2.8 per 1000 live births, and affects 0.5 off all pregnancies..