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.
In contrast, the word intervention tends to be specific and concrete, and thus the word is often countable; for example, one instance of cardiac catheterization is one intervention performed, and coronary care (noncount) can require a series of interventions (count). At the extreme, the piling on of such countable interventions amounts to interventionism, a flawed model of care lacking holistic circumspection—merely treating discrete problems (in billable increments) rather than maintaining health. Therapy and treatment, in the middle of the semantic field, can connote either the holism of care or the discreteness of intervention, with context conveying the intent in each use. Accordingly, they can be used in both noncount and count senses (for example, therapy for chronic kidney disease can involve several dialysis treatments per week). The words aceology and iamatology are obscure and obsolete synonyms referring to the study of therapies. The English word therapy comes via Latin therapīa from Greek: θεραπεία and literally means "curing" or "healing". Types of therapies
Therapy comes in different forms. These include, cognitive behavioral therapy, dialectical behavior therapy, mindful based cognitive therapy, physical therapy, etc. Therapists are here for use and used daily by many people. Therapist are trained to provide treatment to an individual or group. Therapy was invented in the 1800s and the founder was Franz Mesmer, the "Father of Western Psychotherapy". Sigmund Freud then comes into play and shows us the understanding depth of all the different types included in therapy. Therapy is used in many ways to shape and help reform a person.