What is placenta previa?

Placenta previa describes a placement of the placenta low in the uterus, where it covers part or all of the cervix. This positioning blocks the birth canal, which has direct consequences for how pregnancy and delivery are managed.

The condition is detected through transvaginal ultrasound, typically performed during the second or third trimester. Providers categorize placenta previa by degree of cervical coverage: marginal (edge touching the cervix), partial (covering part of the cervix), or complete (fully covering the cervical opening). The position of the placenta is measured in relation to the internal cervical os, the opening into the birth canal.

Why this matters: a placenta covering the cervix creates a physical barrier that must be delivered before the baby can pass through the birth canal. This makes vaginal delivery unsafe, as the placenta would separate and bleed heavily before labor progresses. For this reason, pregnancies with confirmed placenta previa after the second trimester are managed with planned cesarean delivery, typically scheduled at 39 weeks. Patients also need clear instructions to seek immediate care if vaginal bleeding occurs, as bleeding in placenta previa can be heavy and sudden.

Some cases of early placenta previa resolve naturally as the uterus grows and the placenta migrates upward away from the cervix. This is why repeat ultrasound imaging in the third trimester helps confirm whether cesarean delivery is actually necessary. Monitoring and proper delivery planning are essential for maternal and fetal safety.

Providers experienced in high-risk pregnancy care are equipped to manage placenta previa diagnosis and coordinate safe delivery.