How does placenta previa cause preterm delivery?

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.
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.

Related Questions

Diagnosed with partial/marginal placenta previa (25 wks). Precautions? Worried about preterm labor, placenta has not moved up since 17 wk scan.

Try to relax! It's still fairly early to be worried. Most patients with marginal placentas deliver vaginally without incident. If this is your first pregnancy your lower uterine segment will not soften until fairly close to term so the placenta may appear to be low until fairly late. As long as there isn't significant vaginal bleeding you shouldn't worry. Read more...
Early labor unlikely. I recommend in my reports to the referring physicians in this situation that they get an ultrasound in the third trimester to make sure that there is no previa. I would advise one sooner if you devlop any vaginal bleeding. Consultation with your OB physician is importatnt. Generally, placenta previa is of concern during the birth process, but does not in itself predispose to premature delivery. Read more...