Cause of bleeding in placenta previa

As the placenta growth slows down in later months and the lower segment
progressively dilates, the inelastic placenta is sheared off the wall of the lower
segment. This leads to opening up to utero-placental vessels and leads to an 
episode of bleeding. As it is a physiological phenomenon which leads to the
separation of the placenta, the bleeding is said to be inevitable. However,
the separation of the placenta may be provoked by trauma including vaginal
examination, coital act, external version or during high rupture of the membranes.
The blood is almost always maternal, although fetal blood may escape from the
torn villi specially when the placenta is separated during trauma.

The mechanisms of spontaneous control of bleeding are :
1. Thrombosis of the open sinuses.
2. Mechanical pressure by presenting part.
3. Placental infarction.