Placenta previa is a condition in which the placenta implants in the lower uterine segment, covering or lying close to the internal cervical os instead of attaching in the upper uterus where it belongs. It occurs in roughly 1 in 200 to 250 pregnancies and is one of the most important causes of antepartum hemorrhage in the second and third trimesters. Its danger lies in the fact that bleeding is painless and can be sudden and massive, threatening both the pregnant person and the fetus. The hallmark clinical sign is painless, bright-red vaginal bleeding, usually appearing in the late second or third trimester. Because the placenta sits over or near the os, any stretching or thinning of the lower uterine segment, or any disturbance of the cervix, can shear placental vessels and produce hemorrhage. This is the central reason for the single most important nursing rule in placenta previa: never perform a digital vaginal examination when previa is known or suspected, because a finger can perforate the placenta and trigger catastrophic bleeding. Risk increases substantially with prior cesarean delivery, because...