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In vasa previa, membranes that contain blood vessels connecting the umbilical cord and placenta lie across the opening of cervix—the entrance to the birth canal.
Vasa previa is present in about 1 in 2,500 to 5,000 deliveries. It is more likely to occur when certain other abnormalities in the placenta are present.
Normally, blood vessels between the fetus and placenta are contained in the umbilical cord. In vasa previa, some of these blood vessels are located in the membranes that surround the fetus, in the area between the fetus and the opening of the cervix. When the membranes rupture, usually a little before labor starts, these blood vessels can be torn. As a result, the fetus may lose a substantial amount of blood. If bleeding is severe, the fetus may die.
Typically, women have painless vaginal bleeding when the membranes rupture, usually soon after labor starts. The fetus's heart rate is often slow.
Doctors may suspect vasa previa when ultrasonography, routinely done earlier in the pregnancy, detects certain abnormalities in the placenta. Ultrasonography, usually done with a device inserted into the vagina, can show the blood vessels crossing over the opening of the cervix.
If vasa previa is diagnosed before delivery, doctors usually do nonstress testing twice a week to check on the well-being of the fetus (see Monitoring the Fetus). Doctors may suggest hospitalizing women at about 30 to 32 weeks of pregnancy so that the fetus can be monitored continually. Women may be given a corticosteroid to help the fetus's lungs mature.
If labor has not started by 32 to 35 weeks and the fetus's lungs are mature, doctors may suggest scheduling a time for cesarean delivery. An emergency cesarean delivery may be necessary if
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