Placenta previa means “placenta first,” because the placenta is the first thing within the uterine cavity.
In this condition, the placenta implants in the lower uterus, close to or even covering the uterine opening, called the internal cervical os, and it can therefore easily bleed, which usually happens after 20 weeks of gestation.
Normally the placenta implants in the upper uterus, and it's unclear why it implants in the lower uterus.
One hypothesis is that the placenta implants lower down when the endometrium in the upper uterus is not well vascularized.
In fact, endometrial damage from things like a previous cesarean section, an abortion (which could be induced or spontaneous), uterine surgery, and multiparity or multiple pregnancies can decrease vascularization and increase the risk of placenta previa.
In other cases, risk factors for placenta previa include having multiple placentas or a placenta with a larger than normal surface area, which can both happen with twins or triplets, as well as maternal age of 35 years or more, intrauterine fibroids, and maternal smoking.
Placenta previa is classified by how close the placenta is to the cervical os, it can be complete where the placenta completely covers the cervical os; partial where the placenta partially covers the cervical os; or marginal where the edge of the placenta extends to within 2 cm of the cervical os.
As the pregnancy progresses, the lower uterine segment grows, and if the placenta’s in the lower uterus, this growth disrupts the placental blood vessels, which can cause bleeding.
This usually a sudden onset of painless bright red bleeding that typically happens after 20 weeks gestation.