This bleeding often starts mildly and may increase as the area of placental separation increases. Placenta praevia should be suspected if there is bleeding after 24 weeks of gestation. Bleeding after delivery occurs in about 22% of those affected.Women may also present as a case of failure of engagement of fetal head. Cause
The exact cause of placenta previa is unknown. It is hypothesized to be related to abnormal vascularisation of the endometrium caused by scarring or atrophy from previous trauma, surgery, or infection. These factors may reduce differential growth of lower segment, resulting in less upward shift in placental position as pregnancy advances. Risk factors
The following have been identified as risk factors for placenta previa:

Previous placenta previa (recurrence rate 4–8%), caesarean delivery, myomectomy or endometrium damage caused by D&C. Women who are younger than 20 are at higher risk and women older than 35 are at increasing risk as they get older. Women who have had previous pregnancies (multiparity), especially a large number of closely spaced pregnancies, are at higher risk due to uterine damage. Smoking during pregnancy; cocaine use during pregnancy
Women with a large placentae from twins or erythroblastosis are at higher risk. Race is a controversial risk factor, with some studies finding that people from Asia and Africa are at higher risk and others finding no difference. Placental pathology (velamentous insertion, succenturiate lobes, bipartite i.e. bilobed placenta etc.) Baby is in an unusual position: breech (buttocks first) or transverse (lying horizontally across the womb).Placenta previa is itself a risk factor of placenta accreta.
An obstetrician may need to divide the anterior lying placenta. In such cases, blood loss is expected to be high and thus blood and blood products are always kept ready. In rare cases, hysterectomy may be required. Complications
Maternal
Antepartum hemorrhage
Malpresentation
Abnormal placentation
Postpartum hemorrhage
Placenta previa increases the risk of puerperal sepsis and postpartum hemorrhage because the lower segment to which the placenta was attached contracts less well post-delivery. Fetal
IUGR (15% incidence)
Hypoxia
Premature delivery
Death

Epidemiology
Placenta previa occurs approximately one of every 200 births globally. It has been suggested that rates of placenta previa are increasing due to increased rate of Caesarian section. Reasons for regional variation may include ethnicity and diet. Africa
Rates of placenta praevia in sub-Saharan Africa are the lowest in the world, averaging 2.7 per 1000 pregnancies. Despite a low prevalence, this disease has had a profound impact in Africa as it is linked with negative outcomes for both the mother and infant. The most common maternal outcome of placenta praevia is extreme blood loss before or after birth (antepartum hemorrhage and postpartum hemorrhage), which is a major cause of maternal and infant mortality in countries like Tanzania. Risk factors for placenta praevia among African women include prior pregnancies, prenatal alcohol consumption, and insufficient gynecologic care. In North Africa placenta praevia rates occur in 6.4 per 1000 pregnancies. Asia
Mainland China has the highest prevalence of placenta praevia in the world, measuring at an average of 12.2 per 1000 pregnancies. Specifically, placenta praevia is most common in Southeast Asia, though the reason for this has not yet been investigated.