Syncope, commonly known as fainting, or passing out, is a loss of consciousness and muscle strength characterized by a fast onset, short duration, and spontaneous recovery. It is caused by a decrease in blood flow to the brain, typically from low blood pressure. There are sometimes symptoms before the loss of consciousness such as lightheadedness, sweating, pale skin, blurred vision, nausea, vomiting, or feeling warm. Syncope may also be associated with a short episode of muscle twitching. Psychiatric causes can also be determined when a patient experiences fear, anxiety, or panic; particularly before a stressful event usually medical in nature. When consciousness and muscle strength are not completely lost, it is called presyncope. It is recommended that presyncope be treated the same as syncope.Causes range from non-serious to potentially fatal. There are three broad categories of causes: heart or blood vessel related; reflex, also known as neurally mediated; and orthostatic hypotension. Issues with the heart and blood vessels are the cause in about 10% and typically the most serious while neurally mediated is the most common. Heart related causes may include an abnormal heart rhythm, problems with the heart valves or heart muscle and blockages of blood vessels from a pulmonary embolism or aortic dissection among others. Neurally mediated syncope occurs when blood vessels expand and heart rate decreases inappropriately. This may occur from either a triggering event such as exposure to blood, pain, strong feelings or a specific activity such as urination, vomiting, or coughing.
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.