Placenta previa is a major cause of antepartum hemorrhage because the placenta implants low in the uterus, partially or completely covering the cervical os. As the cervix dilates or effaces in late pregnancy, the placental attachment site is disrupted, leading to painless, bright red vaginal bleeding. This bleeding can be significant and poses a risk to both mother and fetus, often necessitating careful monitoring and sometimes cesarean delivery to prevent further hemorrhage. Unlike other causes, placenta previa directly involves the placental site, which is central to antepartum bleeding.

Reference: Williams Obstetrics, Cunningham FG, 25th Edition.