Placenta Previa

Placenta previa occurs when the placenta covers the internal cervical os. A complete previa covers the os entirely; partial and marginal previa cover part of it. A low-lying placenta is close to the os—within 2 cm—but does not cover it. That distinction matters because the placenta’s relationship to the os helps determine whether cervical dilation and vaginal birth can be safe. The lower uterine segment thins and stretches as pregnancy advances. If placental tissue is attached over the os, this stretching can shear the placental attachment and open maternal blood vessels. The cervix may also begin to efface or dilate, disrupting the placental edge. Unlike placental abruption, this bleeding is classically painless because it is not primarily caused by painful separation within a tense, contracting uterus. The typical presentation is sudden, bright-red vaginal bleeding without abdominal pain. Bleeding may stop and recur, and the visible amount may underestimate total blood loss. The mother can become hypovolemic, while the fetus may be affected by reduced uteroplacental perfusion or by the preterm birth that bleeding triggers.