Why does placenta previa cause cramping and bleeding?

Bleeding: Placenta previa is the placenta implanting over the internal opening of the cervix, and is a very serious problem. It will necessitate you having a caesarean section, as you will not be able to have a vaginal birth. As to bleeding, it can be very serious, and should be reported to your doctor immediately.

Placenta previa: Usually bleeding associated with placenta previa is painless. Rarely, if there is separation of placenta and blood is collected retroplacentally, it can be painful.

Blood => irritation: 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.