Vasa Praevia: Diagnosis and Management
Definition
Vasa praevia occurs when fetal blood vessels run through the free membranes, unsupported by placental tissue or Wharton's jelly of the umbilical cord, and cross or run near the internal cervical os below the presenting part. Unprotected, these vessels are vulnerable to rupture when the membranes rupture — spontaneously or artificially — or during labour, causing rapid fetal exsanguination.
- Type I: velamentous cord insertion into the membranes, with the unsupported vessels crossing over or near the os
- Type II: succenturiate or accessory (often bilobed) placental lobe, with connecting vessels running between the lobes across the os
Risk Factors
- Velamentous cord insertion (accounts for the majority of cases)
- Succenturiate or bilobed placenta
- Low-lying placenta or placenta praevia identified on second-trimester ultrasound (present in most cases of vasa praevia found at delivery)
- Multiple pregnancy
- Assisted conception (IVF), which increases the risk of velamentous cord insertion and Type I vasa praevia
- Placenta praevia that appears to resolve/migrate in the third trimester, potentially leaving unsupported vessels behind
Diagnosis
- Routine, universal screening for vasa praevia at the anomaly scan is not currently recommended for all women — the evidence does not support this for a low-risk population.
- In women with identified risk factors (low-lying placenta, succenturiate/bilobed placenta, IVF pregnancy), a combination of transabdominal and transvaginal ultrasound with colour Doppler at the routine fetal anomaly scan gives high diagnostic accuracy with a low false-positive rate.
- Transvaginal ultrasound with colour Doppler imaging in the second trimester is highly sensitive for detecting vasa praevia.
- Because vasa praevia identified in the second trimester can resolve as the placenta grows/migrates (roughly one-fifth of cases resolve before delivery), a follow-up scan later in the third trimester is suggested to confirm the diagnosis persists, particularly where a low-lying placenta is present.
- Intrapartum, vasa praevia may present as pulsating fetal vessels felt on vaginal examination within the membranes over the os, or as sudden dark-red vaginal bleeding with acute fetal compromise — the classic triad is membrane rupture (spontaneous or artificial) + painless vaginal bleeding + fetal heart rate abnormality or fetal death.
Management
- Antenatal ultrasound diagnosis followed by planned caesarean section is associated with markedly improved survival compared with cases first discovered when membranes rupture in labour, where fetal mortality is substantial because vessel rupture leads to rapid exsanguination.
- Corticosteroids for fetal lung maturation should be offered from around the early third trimester, given the elevated risk of preterm and emergency delivery.
- Prophylactic hospitalisation from around the early-to-mid third trimester should be individualised, weighing factors such as multiple pregnancy, antepartum bleeding, threatened preterm labour, and distance/access to the delivery unit — it is not mandated for every woman with confirmed vasa praevia.
- Planned caesarean section, timed in the late preterm/early term window, should be carried out electively before the onset of labour to avoid spontaneous membrane rupture.
- If preterm prelabour rupture of membranes, bleeding, or labour occurs at a viable gestation, caesarean section should be performed without delay as an emergency.
- The overarching management goal is to deliver before membrane rupture occurs, while minimising the harms of iatrogenic prematurity from an unnecessarily early planned delivery.
High-Yield Exam Points
- Classic presentation: membrane rupture + painless vaginal bleeding + acute fetal compromise (bradycardia or fetal death) — fetal blood, not maternal, is being lost.
- Two types: Type I (velamentous cord insertion) and Type II (succenturiate/bilobed placenta with bridging vessels).
- Universal screening is not recommended; targeted TA + TV colour Doppler is used in women with risk factors (low-lying placenta, succenturiate lobe, IVF).
- Antenatal diagnosis + elective pre-labour caesarean section is the key intervention that converts a high-mortality emergency into a good-outcome delivery.
- Antenatal corticosteroids and the decision on prophylactic admission are both timed in the third trimester and individualised to risk (bleeding, multiple pregnancy, access to care).
- Distinguish from placenta praevia (GTG 27a): vasa praevia concerns unsupported fetal vessels over the os, not placental tissue — but the two conditions share overlapping risk factors and often coexist.
Source: RCOG Green-top Guideline No. 27b (2018 (published 27 September 2018, Jauniaux et al., BJOG 126(1):e49–e61))
Read the original on rcog.org.uk
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