Placenta previa is the extension of placental tissue over the endocervical os, which typically presents as an incidental finding on the 18- to 20-week anatomy ultrasound or as painless vaginal bleeding in the second or third trimester. Occurrence is up to 2% at approximately 20 weeks' gestation.

The bleeding associated with placenta previa may have no identifiable cause or may be provoked by a recent vaginal examination, intercourse, or labor. Sometimes, blood can infiltrate the wall of the uterus (Couvelaire uterus). On speculum examination, the placenta may be visualized in addition to varying amounts of bleeding. To avoid massive hemorrhage, a digital examination should never be performed.

Most cases of placenta previa are diagnosed in the second trimester during routine ultrasound examination. Placenta previa diagnosed early in the second trimester (20-25 gestational weeks) is associated with a high rate of resolution in the third trimester (87.5% of cases). This is likely due to atrophy of the placenta near the cervical os and lengthening of the lower uterine segment throughout the pregnancy. However, placenta previa that persists until the time of delivery is associated with adverse maternal and fetal outcomes, including maternal hemorrhage, preterm birth, lower birth weight, and neonatal respiratory distress syndrome.

Placenta previa may be complicated by placenta accreta spectrum, which includes accreta (the attachment of the placental trophoblast to the myometrium), increta (invasion of the myometrium), and percreta (invasion beyond the myometrium and into surrounding organs such as the bladder).

Major risk factors include prior history of cesarean delivery, prior history of placenta previa, and multiparity. Additional risk factors include smoking, maternal age older than 35 years, use of assisted reproductive technology, and a previous uterine surgical procedure. Uterine scarring due to a prior uterine procedure may promote abnormal placentation near the endocervix.