Vasa previa occurs when membranes that contain fetal blood vessels connecting the umbilical cord and placenta overlie the internal cervical os.
Vasa previa can occur on its own or with placental abnormalities, such as a velamentous cord insertion. In velamentous cord insertion, vessels from the umbilical cord run through part of the chorionic membrane rather than directly into the placenta (see Figure: Vasa previa.). Thus, the blood vessels are not protected by Wharton jelly within the cord, making fetal hemorrhage more likely to occur when the fetal membranes rupture.
Prevalence is about 1/2500 to 5000 deliveries. Fetal mortality rate may approach 60% if vasa previa is not diagnosed before birth.
The diagnosis should be suspected based on presentation or results of routine prenatal ultrasonography. At presentation, the fetal heart rate pattern, commonly sinusoidal, is usually nonreassuring. The diagnosis is typically confirmed by transvaginal ultrasonography. Fetal vessels can be seen within the membranes passing directly over the internal cervical os. Doppler color flow mapping can be used as an adjunct. Vasa previa must be distinguished from funic presentation (prolapse with the umbilical cord between the presenting part and the internal cervical os), in which fetal blood vessels wrapped with Wharton jelly can be seen covering the cervix.
Antenatal management of vasa previa is controversial, partly because randomized clinical trials are lacking. At most centers, nonstress testing is done twice a week beginning at 28 to 30 wk. The purpose is to detect compression of the umbilical cord. Admission for continuous monitoring or for nonstress testing every 6 to 8 h at about 30 to 32 wk is often offered. Corticosteroids may be used to accelerate fetal lung maturity.
If premature rupture of the membranes occurs, vaginal bleeding continues, or fetal status is nonreassuring, emergency cesarean delivery is usually indicated. If none of these problems are present and labor has not occurred, clinicians can offer to schedule cesarean delivery; tests to assess fetal lung maturity (which usually occurs between 32 and 35 wk) may be done to determine the timing of delivery.