The cause of placenta previa is not known; however, certain risk factors seem to increase the incidence of this occurring. Scarring from uterine surgery reduces the vascularity of the upper uterine segment (induced abortion, previous cesarean sections, past molar pregnancy, and fibroid tumors). Factors that increase risk include previous placenta previa, multiple fetal gestation (because multiple fetal gestation requires a larger surface area for placental implantation), maternal age (over 35 years), smoking, cocaine use, closely spaced pregnancies, and multiparity.

Placenta previa is usually diagnosed after the onset of sudden painless vaginal bleeding during the second or third trimester of pregnancy. Also, bleeding can occur when there is stretching and thinning of the lower uterine segment during the third trimester. It could start when a woman is resting or during any activity. The bleeding is bright red and can be intermittent, occur in gushes, or, less commonly, be continuous. Because a pregnant woman can lose up to 40% of blood volume without showing signs of shock (heavy blood loss included) and her vital signs can be normal, the most accurate assessment of this problem is decreasing urinary output. Vigorous fetal heart rate assessment is necessary to ensure the integrity of fetal health status. Vaginal examinations or other methods to stimulate uterine contractions should be omitted until the physician further assesses the pregnant woman.