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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.
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]
If placenta previa is present at the time of delivery, vaginal delivery is contraindicated because the placenta is blocking the fetus's passageway to the vaginal canal. Herpes simplex virus with active genital lesions or prodromal symptoms is a contraindication for vaginal delivery so as to avoid mother-fetal transfer of HSV lesions.
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.
These factors are known to be the primary pathophysiology cause placental disease. Which is considered to be associated with more than half of premature births. [4] Abnormalities present within the spiral arteries lead to higher velocities in blood, in turn causes the maternal villi to shred. [5]
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]
During the third trimester of pregnancy, thinning of the lower uterine segment or contractions caused by cervical dilation can increase the amount of stress placed on the placental attachment to the uterine wall. In patients with placenta praevia, these stresses can cause detachment of the placenta from the uterine wall causing haemorrhaging.
There are several posited ways that have been positioned to cause amniotic fluid embolism. The first of which involves the thought that a combination or one of the following that include a difficult labor, a placenta that is abnormal and trauma to the abdomen through a caesarean section or other surgical tools dissipates the barrier that exists from the maternal fluid to the fetal fluid.