Vasa praevia

Vasa praevia is the presence of fetal blood vessels running in the membranes in close proximity to the internal os (within 2 cm). It is described as type 1 when it occurs in association with velamentous insertion of the umbilical cord and type 2 when it occurs in associ­ation with a bilobed placenta or succenturiate lobe.

Placental abnormalities such as placenta praevia, velamentous in­sertion of the cord, bilobed placenta, succenturiate lobe, multiple pregnancy, and in vitro fertilization are known risk factors for vasa praevia (72).

Diagnosis

Although routine screening for vasa praevia is not recommended, a high index of suspicion should be maintained in patients with multiple risk factors linked to vasa praevia. If vasa praevia is sus­pected in the antenatal period, the diagnosis can be confirmed by transvaginal ultrasound with colour Doppler. This typically shows fetal vessels running in close proximity to the internal cervical os. This should be differentiated from cord presentation, where the ves­sels are surrounded by Wharton's jelly, and the cord typically floats away when the uterus is gently pushed (73). When vasa praevia is in­cidentally diagnosed in the second trimester, further assessment by transvaginal ultrasound and colour Doppler should be performed in the third trimester as up to 15% of the cases will resolve by the third trimester (74).

A presumptive diagnosis of vasa praevia is made when brisk bleeding and fetal distress occur immediately after spontaneous or artificial rupture of membranes, particularly in patients with known associations.

Management

When vasa praevia has been diagnosed on a routine ultrasound scan in the antenatal period, fetal surveillance is recommended and ante­natal corticosteroids are recommended up to 34 weeks' gestation because of the increased likelihood of preterm delivery. Elective in­patient monitoring from 30 to 34 weeks has been advocated to allow closer fetal surveillance with the added possibility of performing caesarean delivery promptly if labour starts or the membranes rup­ture (75). Elective delivery by caesarean section is recommended be­tween 35 and 37 weeks once a course of steroid has been completed (31, 76). When there is brisk bleeding and sudden fetal distress or there is a sinusoidal fetal heart rate pattern following ruptured mem­branes, prompt delivery by caesarean section should be considered with a provisional diagnosis of vasa praevia. There is usually no time to perform a confirmatory test for fetal bleeding (e.g. Apt test or Kleihauer-Betke test) because vasa praevia causes rapid fetal exsan­guination and it is imperative to act on the clinical diagnosis to save the fetus in the acute intrapartum scenario.