Placenta Previae

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Submitted By immordee
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Placenta previa involves implantation of the placenta over the internal cervical os. Variants include complete implantation over the os (complete placenta previa), a placental edge partially covering the os (partial placenta previa) or the placenta approaching the border of the os (marginal placenta previa). A low-lying placenta implants in the caudad one half to one third of the uterus or within 2-3 cm from the os.

See Medscape's Pregnancy Resource Center.
Placental implantation is initiated by the embryo (embryonic plate) adhering in the lower (caudad) uterus. With placental attachment and growth, the developing placenta may cover the cervical os. However, it is thought that a defective decidual vascularization occurs over the cervix, possibly secondary to inflammatory or atrophic changes. As such, sections of the placenta having undergone atrophic changes could persist as a vasa previa.
A leading cause of third trimester hemorrhage, placenta previa presents classically as painless bleeding. Bleeding is thought to occur in association with the development of the lower uterine segment in the third trimester. Placental attachment is disrupted as this area gradually thins in preparation for the onset of labor. When this occurs, bleeding occurs at the implantation site as the uterus is unable to contract adequately and stop the flow of blood from the open vessels. Thrombin release from the bleeding sites promotes uterine contractions and a vicious cycle of bleeding-contractions-placental separation-bleeding.
United States
Placenta previa occurs in 0.3-0.5% of all pregnancies. The risks increase 1.5- to 5-fold with a history of cesarean delivery. With an increased number of cesarean deliveries, this risk can be as great as 10%. Recent studies show that a previous cesarean delivery did not increase the odds of…...

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