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Vasa previa: A condition in which blood vessels within the placenta or the umbilical cord are trapped between the fetus and the opening to the birth canal, a situation that carries a high risk the fetus may die from hemorrhage due to a blood vessel tearing at the time the fetal membranes rupture or during labor and delivery. Another danger is lack of oxygen to the fetus.

Vasa previa often occurs with a low-lying placenta (due to scarring of the uterus by a previous miscarriage or a D&C), an unusually formed placenta (a bilobed placenta or succenturiate-lobed placenta), an in-vitro fertilization pregnancy, and multiple pregnancies (twins, triplets, etc). Vasa previa also may accompany velamentous insertion of the umbilical cord.

Vasa previa may not be suspected until the fetal vessel rupture occurs. Reduction in fetal mortality depends on prenatal diagnosis. When vasa previa is found before labor, the baby has a much greater chance of surviving. Vasa previa can be detected during pregnancy as early as the 16th week of pregnancy with use of transvaginal sonography in combination with color Doppler.

When vasa previa is diagnosed, C-section before labor begins can save the baby's life. The C-section should be done early enough to avoid an emergency, but late enough to avoid problems associated with prematurity.

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