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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.
Preterm birth is the most common cause of death among infants worldwide. [1] About 15 million babies are preterm each year (5% to 18% of all deliveries). [2] Late preterm birth accounts for 75% of all preterm births. [18] This rate is inconsistent across countries.
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.
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]
The main causes of obstructed labour include a large or abnormally positioned baby, a small pelvis, and problems with the birth canal. [2] Both the size and the position of the fetus can lead to obstructed labor. Abnormal positioning includes shoulder dystocia where the anterior shoulder does not pass easily below the pubic bone. [2]
Placental abruption causes blood loss from the mother and loss of oxygen and nutrients to the placenta occasionally leading to preterm labour. [14] Other causes of placental abruption can be abdominal trauma or sudden decompression of amniotic fluid , however it is not uncommon for the cause of placental abruption to be unknown.
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.