Vasa praevia refers to a situation where there are aberrant fetal vessels crossing over or in close proximity to the internal cervical os, ahead of the fetal presenting part. These vessels are within the amniotic membranes, without the support of the placenta. Vasa praevia is a rare but potentially catastrophic cause of antepartum haemorrhage.

Epidemiology

Vasa praevia occurs in 0.6 per 1000 pregnancies ¹¹.

Risk factors

• multiple gestations

• low lying placenta

• succenturiate lobe or bilobed placenta

• IVF pregnancy

Clinical presentation

Prior to rupture of the amniotic membrane during labour, compression of the vulnerable fetal blood vessels by the presenting part may lead to fetal heart decelerations and bradycardia.

However once ruptured, vasa praevia will result in brisk vaginal bleeding with rapid fetal exsanguination.

Pathology

Vasa praevia can be of two types:

• type I (~90% ³): abnormal fetal vessels connect a velamentous cord insertion with the main body of the placenta

• type II

  • abnormal vessels connect portions of a bilobed placenta

  • placenta with a succenturiate lobe: due to this association, vasa praevia needs to be excluded in patients with variant placental morphology

These vessels are unsupported by Wharton jelly or placental tissue and are at risk of rupture during labour.

Radiographic features

Ultrasound

Sonographic features are considered generally specific (~90%) ².

The diagnosis is often made with transabdominal colour Doppler sonography demonstrating flow within vessels that are seen overlying the internal cervical os. Greyscale images may suggest the diagnosis if there are echogenic parallel or circular lines within the placenta near the cervix.

Occasionally a transvaginal scan is required to better visualise aberrant vessels. Transvaginal ultrasound has a reported sensitivity of 100% and specificity of 99-99.8% when performed with colour Doppler ¹².

Treatment and prognosis

If recognised antenatally, vasa praevia usually requires an elective caesarean section to avoid the risk of complications during vaginal delivery.

If recognised intrapartum, an emergency caesarean section is usually performed.

Differential diagnosis

On antenatal ultrasound consider:

• marginal placenta praevia: where prominent maternal vessels are appreciated at the edge of the placenta

• funic presentation