Miscarriage it is estimated that as many as 50% of all zygotes are lost before delivery. Most are lost before implantation. Approximately 15%  of all pregnancies end in miscarriage, or spontaneous abortion, which results from the death or early delivery of the fetus. Before about 24 weeks post- LMP the fetus is not viable outside the uterus. After 24 weeks, but before 37 weeks post-LMP, the infant is referred to as premature. Although there is a higher incidence of birth defects among aborted fetuses, the vast majority of miscarried fetuses appear to be normal. Many factors can cause a miscarriage, many of which do not directly involve the fetus, and many of which are unknown. One common cause of miscarriage is improper implantation of the blastocust in the uterus. In most cases, the blastocyst implants in the upper part of the uterus, but occasionally a blastocyst can implant near the opening into the cervical canal, a condition called placenta previa. As the fetus grows and the uterus stretches, the previa placenta may tear away from the uterine wall, a condition called placental abruption. In fetus often dies. The associated hemorrhaging can be life-threatening to the mother as well.