Vasa previa is a condition in which the intramembranous fetal blood vessels cross the internal os. It has a high fetal mortality because of the hemorrhage that occurs when these vessels are torn at the time of labor, delivery or when the membranes rupture. Vasa previa often remains unsuspected until this fatal fetal vessel rupture occurs. The reduction in the fetal mortality from this condition depends on its antenatal diagnosis. In one very big series^[1], this condition was suspected in 18 pregnancy among 93,874 second and third trimester pregnancies and confirmed in 15, which means the incidence was of about 1.6:10,000 pregnancies. In another (prospective) study^[2], vasa previa was diagnosed in two cases out of 586 (1 in 293 pregnancies). So, the incidence varies from one series to another one, being considered for some to be around 3:10,000 pregnancies^[3].

Ultrasonographic findings:

Vasa previa must be ruled out in all cases of bilobed, succenturiated, and low-lying placentas, pregnancies resulting from in vitro fertilization, and multiple pregnancy^[4]. In these cases it is important to look for vessels near the cervix. The diagnosis can be confirmed by Doppler and endovaginal studies. A common pitfall with Doppler is flash artifact that mimics a vessel near the internal os. Simply repeating the image is sufficient to exclude the artifact.

Case 1

Fig. 2 to 5: Color Doppler detects the intramembranous vessels at the cervical os more readily.

Case 2

Fig. 6 and 7: Another case in which color Doppler and endovaginal scanning confirm the presence of aberrant vessels over the internal cervical os.

Case 3

Fig. 8 and 9: In this case, Doppler was not available, but the velamentous insertion of the umbilical cord was noticed across the internal cervical os, at 23 and 31 weeks of pregnancy.

Fig. 10 to 13: Postnatal confirmation of the velamentous insertion responsible for the vasa previa.

Flash artifact

Flash artifact that mimics a vessel near the internal os. Note that on some image the flow is going in one direction while in the other the direction is reversed. To exclude this artifact spend a few more minutes to make sure it persists. Or drop a pulse-wave sample on the suspicious "flow†to check that although it has color it does not have pulsations as a fetal vessel would have.

References

^[1] Lee W, Lee VL, Kirk JS, Sloan CT, Smith RS, Comstock CH Vasa previa: prenatal diagnosis, natural evolution, and clinical outcome. Obstet Gynecol 2000 Apr;95(4):572-6

^[2] Nomiyama M, Toyota Y, Kawano H Antenatal diagnosis of velamentous umbilical cord insertion and vasa previa with color Doppler imaging. Ultrasound Obstet Gynecol 1998 Dec;12(6):426-9

^[3] Chen KH, Konchak P Use of transvaginal color Doppler ultrasound to diagnose vasa previa. J Am Osteopath Assoc 1998 Feb;98(2):116-7

^[4] Oyelese KO, Turner M, Lees C, Campbell S Vasa previa: an avoidable obstetric tragedy. Obstet Gynecol Surv 1999 Feb;54(2):138-45