Why is there bleeding with placenta previa?

Placenta: When placenta is on lower uterine segment, it is called placenta previa. When plans is completely covering the internal os it is complete placenta previa. When the lower segment of uterus gets stretched in last trimester, it distrupts the placenta to cause bleeding.

Maternal vessels.: Placenta previa means that the internal cervical os is covered by trophoblast. When labor occurs, or the amniotic membranes rupture, the placenta starts detaching from the lower uterus, so the maternal blood circulating through the placenta exits from its proper place and manifests as vaginal bleeding (not fetal blood).

Leaky vessels: Previa: Placenta overlies the cervix/birth canal. Gr 1 means placenta is close, Gr 4 means it's directly overlying/blocking the baby's exit -- high chance of preterm labor & bleeding; you'd need a C/S at 36-37wks. More common in woman w/ hx of C/S & smoking. Not much you can do but wait. If dx'ed early, the uterus still grows/stretches and a placenta can migrate away even in 3rd tri. Follow w/ US.