How does placenta previa cause preterm delivery?

In placenta previa,: the placenta either partially or completely overlaps the opening of the cervix. As the cervix starts to dilate, the blood vessels of the placenta are exposed and start to bleed, and if the bleeding doesn't stop or is very heavy, the baby needs to be delivered immediately or may not survive due to blood loss. The cervix softens several weeks before due date, so important to monitor carefully.

Bleeding and Cntrxns: 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.