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The following have been identified as risk factors for placenta previa: Previous placenta previa (recurrence rate 4–8%), [13] caesarean delivery, [14] myomectomy [10] or endometrium damage caused by D&C. [13] Women who are younger than 20 are at higher risk and women older than 35 are at increasing risk as they get older.
Placenta previa is a condition that occurs when the placenta fully or partially covers the cervix. [13] Placenta previa can be further categorized into complete previa, partial previa, marginal previa, and low-lying placenta, depending on the degree to which the placenta covers the internal cervical os.
The placenta may bleed or begin to separate early from the wall of the uterus. [28] It is normal for the placenta to separate from the uterine wall during delivery, but it is abnormal for it to separate prior to delivery; this condition is called placental abruption and can be dangerous for the fetus. [29]
Along with placenta previa and uterine rupture it is one of the most common causes of vaginal bleeding in the later part of pregnancy. [6] Placental abruption is the reason for about 15% of infant deaths around the time of birth. [2] The condition was described at least as early as 1664. [7]
Besides placenta previa and placental abruption, uterine rupture can occur, which is a very serious condition leading to internal or external bleeding. Bleeding from the fetus is rare, but may occur with two conditions called vasa previa and velamentous umbilical cord insertion where the fetal blood vessels lie near the placental insertion site unprotected by Wharton's jelly of the cord. [11]
Ischemic placental disease leads to the attachment of the placenta to the uterine wall to become under-perfused, causing uteroplacental ischemia. Where the term overarches the pathology associated with preeclampsia , placental abruptions and intrauterine growth restriction (IUGR). [ 3 ]
Placenta praevia or vasa praevia refers to the placenta or fetal blood vessels, respectively, covering or being located close to the opening of the uterus. More than half of women affected by placenta praevia (51.6)% have bleeding before delivery. [12] Vasa praevia occurs in about 0.6 per 1000 pregnancies. [13]
Abnormal development of the placenta leads to poor placental perfusion. The placenta of women with pre-eclampsia is abnormal and characterized by poor trophoblastic invasion. [26] It is thought that this results in oxidative stress, hypoxia, and the release of factors that promote endothelial dysfunction, inflammation, and other possible reactions.