Prevalence:

• 1 in 3,000 pregnancies.
  

Ultrasound diagnosis:

• Umbilical vessels, unsupported by either the umbilical cord or placental tissue, traverse the fetal membranes of the lower segment above the cervix.
• Use of transvaginal ultrasound and color Doppler are necessary to make the diagnosis.

Associated abnormalities:

• Vasa previa usually occurs in association with velamentous cord insertion, bipartite placenta, or succenturiate lobe, where vessels run through the membranes to join the separate lobes.
• Risk factors are multiple pregnancies, IVF conceptions (1 in 300) and low lying placenta in the second trimester.

Investigations:

• Detailed ultrasound examination, including transvaginal sonography with color Doppler.

Follow up:

• Serial scans every 2 weeks after 26 weeks to monitor cervical length. If there is cervical shortening (<25 mm) hospitalization should be considered.

Delivery:

• Delivery by cesarean section at 34 to 36 weeks, depending on cervical length.

Prognosis:

• Fetal death: >60% if not diagnosed prenatally and <3% if diagnosed prenatally.

Recurrence risk:

• No increased risk of recurrence.