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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.
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.
This usually a sudden onset of painless bright red bleeding that typically happens after 20 weeks gestation.
Placenta previa is a pregnancy complication in which the placenta implants in the lower uterus and partially or fully covers the internal cervical os, making vaginal delivery difficult or impossible. It can cause heavy vaginal bleeding and a serious threat to both the mother and the fetus. Symptoms include painless vaginal bleeding in the third trimester, and some risk factors are previous placenta previa, multiple gestation, and uterine fibroids.
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