Vasa Previa Defined
Vasa previa is a rarely reported condition in which exposed fetal vessels traverse the amniotic membranes between the baby’s presenting part and the internal cervical os, unprotected by placental tissue or umbilical cord. The incidence of vasa previa has been estimated at 1 in 2500 births, although has been reported to vary between 1:513 and 1:6000 in naturally conceived pregnancies, and as high as 1:293 in IVF-assisted pregnancies. The condition has a high fetal mortality rate (50-95%) when undiagnosed prenatally. This can be attributed to rapid fetal exsanguination resulting from the vessels tearing when the cervix dilates, membranes rupture or if the vessels become pinched off as they are compressed between the baby and the walls of the birth canal.
The aberrant vessels result from:
Velamentous Insertion of the Umbilical Cord
Bilobed and Trilobed Placenta
Succenturiate Lobed Placenta
Pathology
Little is known about the cause of these conditions. The most widely recognized theory is called trophotropism. According to
Dr. Harris Finberg, trophotropism in placental tissue can be compared to the tendency of a plant to lean towards the sun to get the
light it needs to survive. Since the lower segment of the uterus is not as nourishing as the upper segment, the placenta will remodel
itself upwards to reach more nourishing tissue.
As the placenta remodels, new growth may occur away from the location where the cord inserts into the placental resulting in
velamentous cord insertion. Or the remodeling may leave the placenta in lobes connected by unprotected blood vessels running
through the membranes between the lobes (bi-lobed, succenturiate lobed placenta).
Vasa previa can result from low-lying placenta or placenta previa, where the placenta is in front of the birth canal.
Risk Factors
Vasa previa might be present if any of the following conditions exist:
- velamentous cord insertion
- bilobed placenta
- succenturiate-lobed placenta
- low-lying placenta or placenta previa (even if it corrects itself!)
- pregnancies resulting from in-vitro fertilization
- multiple pregnancies
- maternal history of D&C or uterine surgery
Management
When vasa previa is detected prior to labor, the baby has a much greater chance of surviving. Vasa previa can be detected during pregnancy with use of transvaginal sonography, preferably in combination with color Doppler. Women with identified risk factors should have this test to rule out vasa previa.
When vasa previa is diagnosed, elective delivery by cesarean section before labor begins can save the baby's life. Ideally, it should be performed early enough to avoid an emergency, but late enough to avoid problems associated with prematurity. The IVPF recommends complete pelvic rest, hospitalization in the 3rd trimester, delivery by approximately 35-36 weeks gestation, and immediate blood transfusion of the infant in the event of a rupture.
Steroid treatments can help accelerate the maturation of the baby's lungs if born prematurely.
When there is bleeding during pregnancy, investigation for the source of the blood is necessary. If the blood is determined to be
fetal (from the baby), immediate action must be taken to assess the condition of the baby.
While these recommendations are based on the best available published studies, every case of vasa previa can be slightly different and it is important that an individual management plan is developed between the mother and her doctors based around these recommendations.
When vasa previa is identified during the second trimester (20 weeks or earlier) there is a small possibility that vasa previa may not persist to term. This can be due to the formation of the lower segment of the uterus in the third trimester which can result in the vessels being located further away from the internal cervical os. This does not necessarily mean that the vessels are far enough from the cervix to allow for a safe natural delivery as the exposed vessels still pose a danger from compression or rupture. There is no established evidence regarding what distance is considered “safe” to deliver naturally when exposed fetal vessels are in the vicinity of the cervical os.