الفهرس 
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Abstract
Placenta Previa refers to the presence of placental tissue that extends over the internal cervical os during pregnancy. (Romeo et al., 2019) The incidence of this condition is reported to be 2% at 20 weeks of
gestation, and through the process of placental migration known as
trophotropism, decreases to around 4–6 per 1000 births between 34 and
39 weeks. (Balayla et al., 2019)
The primary risk factors for the development of placenta Previa
include a prior history of placenta Previa, previous cesarean delivery, multiple
gestations, use of fertility treatments and increasing maternal age, among
others. The risk of recurrence in subsequent pregnancies is reported at 4–8%.
(Adere et al., 2020)
Given its location over the cervical os, a proportion of the placental
surface is exposed and lacks a proper utero-placental interphase. Wellestablished
sequelae of this condition include the potential for severe antenatal
bleeding and preterm birth, as well as the need for cesarean delivery.
(Sommerstein et al., 2020)
The risk of bleeding is thought to occur when uterine contractions or
gradual changes in the cervix and lower uterine segment apply shearing forces
to the inelastic placental attachment site, resulting in partial detachment. The
risk of placenta Previa increases after a single Cesarean delivery and rises
further with increasing number of Cesarean deliveries. (Chen et al., 2020)
Epidemiological data suggest that the scar left following a Cesarean
delivery in the myometrium of the lower uterine segment encourages both implantation of the blastocyst in the area of the scar and abnormal adherence
or invasion of placental villi within the scar tissue. (Jauniaux et al., 2020)
Poor vascularization and tissue oxygenation in the area of a Cesarean
scar is associated with local failure of re-epithelialization and
decidualization, which has an impact on both implantation and placentation, as
well as a possible effect on placental development and, subsequently, fetal
growth. (Jauniaux et al., 2019).