placenta previaTy

Placenta praevia (placenta previa ) is an obstetric complication in which the placenta is attached to the uterine wall close to or covering the cervix It can sometimes occur in the later part of the first trimester, but usually during the second or third. It is a leading cause of antepartum  bleeding). It affects approximately 0.5% of all labour 

It is hypothesized^] to be related to abnormal vaskularisation of caused by scarring or atropy  from previous trauma, surgery, or infection.

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 it, so there is no bleeding. If the placenta does overlie the lower segment, it may shear off and a small section may bleed.