Vasa previa can occur on its own (see figure Vasa previa) 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. 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 of vasa previa 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 or near 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. In funic presentation, unlike in vasa previa, the umbilical cord moves away from the cervix during ultrasound evaluation; in vasa previa, the cord is fixed in place.
Prenatal 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 weeks. The purpose is to detect compression of the umbilical cord. Admission for continuous monitoring or for nonstress testing every 6 to 8 hours at about 30 to 32 weeks is often offered.
Corticosteroids are used to accelerate fetal lung maturity.
Emergency cesarean delivery is usually indicated if any of the following occurs:
If none of these problems are present and labor has not occurred, clinicians can offer to schedule cesarean delivery. Cesarean delivery between 34 to 37 weeks has been suggested, but the timing of delivery is controversial; some evidence favors delivery at 34 to 35 weeks.
Vasa previa may be accompanied by other placental abnormalities, such as velamentous insertion, which increases the risk of fetal hemorrhage when the fetal membranes rupture.
Suspect vasa previa based on symptoms and (painless vaginal bleeding, rupture of membranes, fetal bradycardia) and/or findings during routine prenatal ultrasonography.
Use transvaginal ultrasonography to confirm vasa previa and to distinguish it (fixed umbilical cord) from funic presentation (movable cord).
Check for compression of the cord using nonstress testing, possibly twice a week beginning at 28 to 30 weeks.
Schedule cesarean delivery, or if premature rupture of the membranes occurs, vaginal bleeding continues, or fetal status is nonreassuring, do emergency cesarean delivery.