Vasa praevia

Definition

• Vasa praevia means fetal vessels running unprotected over or close to the internal os.
• The usual cause is a velamentous cord insertion or a succenturiate lobe with connecting vessels.
• Risk factors are a low-lying placenta, multiple pregnancy, assisted reproduction and accessory lobes.

Classification

• Type I: the unprotected vessels arise from a velamentous cord insertion.
• Type II: the vessels connect the main placenta with an accessory lobe (succenturiate or bilobed placenta) and cross the internal os.

Occurrence & epidemiology

Epidemiology

• Vasa praevia is rare.
• It is considerably more common after assisted reproduction and in multiple pregnancies.

Clinical features

• Undiagnosed vasa praevia is often asymptomatic until the membranes rupture.
• If the unprotected vessels tear at membrane rupture, painless vaginal bleeding from the fetal circulation follows, with rapid fetal bradycardia because the fetal blood volume is small.
• Compression of the vessels by the presenting part can also cause fetal heart rate changes.

Diagnosis

• It is made transvaginally with colour Doppler, often incidentally while looking at the cervix.
• The differential includes a cord loop in the lower segment, which moves when she changes position.

Imaging findings

• Transvaginal colour Doppler ultrasound shows vessels running over or immediately next to the internal os, with a fetal arterial or venous flow pattern on pulsed Doppler.

Keep learning in the app

Further reading (selection)

1. Obstetrics & Gynecology (2023): Vasa Previa (PubMed Central)
2. StatPearls: Placenta Abnormalities (NCBI Bookshelf)
3. DocCheck Flexikon, Vasa praevia

Cross-references