Placenta previa is an obstetric complication in which the placenta is inserted partially or wholly in the lower uterine segment. It is a leading cause of antepartum haemorrhage (vaginal bleeding). It affects approximately 0.4-0.5% of all labours.
In the last trimester of pregnancy the isthmus of the uterus unfolds and forms the lower segment. In a normal pregnancy the placenta does not overlie. If the placenta does overlie the lower segment, as is the case with placenta previa, it may shear off and a small section may bleed.