Vasa praevia: when to look closer

by Ben Sweetlove | Jul 30, 2026 | Pregnancy, QUFW News

Vasa praevia is a near-perfect example of a condition where ultrasound can genuinely change the outcome of a pregnancy. Vasa Praevia can be silent; the baby gives no warning, and the only thing standing between a normal birth and a preventable tragedy is early diagnosis in experienced hands.

Patients diagnosed with vasa praevia have emphasised the importance of receiving clear, evidence-based information about the risks and benefits of different care pathways. The primary benefit of screening for vasa praevia is that antenatal diagnosis significantly increases the likelihood of perinatal survival.

This post summarises current best practice, drawing on the RANZCOG guideline Vasa Praevia (C-Obs 47), Version 3 (July 2025), the most relevant evidence-based resource for practice in Australia and Aotearoa New Zealand. Written for referring GPs and clinicians, it covers what vasa praevia is, why it matters so much at delivery, the histories and findings that should raise your index of suspicion, and when targeted specialist assessment is worth arranging. (You may also see it written as vasa previa, the spelling used in much of the international literature; the two terms describe the same condition.)

Table of contents

1. What is vasa praevia?
2. Why diagnosis matters
3. Who is at risk?
4. How is vasa praevia diagnosed?
5. When to refer to a tertiary unit
6. Delivery and management
7. Refer your patient to QUFW

What is vasa praevia?

Vasa praevia is the presence of unprotected fetal blood vessels within the amniotic membranes over or near the cervical os. Rupture of the membranes before or during labour may result in fetal death from rapid blood loss if an unprotected fetal blood vessel near the cervix is damaged.

Three types are described:

• Type 1 – characterised by a velamentous insertion of the umbilical cord onto the membranes distant to the placenta, with the vessels traversing the membranes over the cervix before reaching the placenta.
• Type 2 – characterised by a succenturiate, multilobed or bilobed placenta with fetal vessels connecting the lobes overlaying the cervix.
• Type 3 – characterised by abnormal fetal blood vessels branching off the placenta and lying unprotected near the cervical os, arising after resolution of placenta praevia or low-lying placenta.

Why diagnosis matters

Screening programs for vasa praevia have been shown to be highly sensitive. When vasa praevia is not diagnosed before labour, the estimated perinatal mortality is between 44% and 70%, and that mortality persists even with an urgent caesarean once a vessel has ruptured. The classic intrapartum presentation is painless dark-red vaginal bleeding after rupture of membranes, accompanied by acute fetal compromise. By the time this happens, the window to act is narrow.

When the diagnosis is made antenatally, and a planned caesarean is performed before labour, perinatal survival is approximately 97-100%. Pooled data show survival of around 98.6% with a prenatal diagnosis versus 72.1% without – roughly a 25-fold higher risk of fetal or perinatal death when the condition is missed, and a markedly higher risk of hypoxic morbidity in survivors.

In short, the condition is almost entirely survivable if found in time, and frequently fatal if not. That dichotomy is what justifies screening and vigilance.

Who is at risk?

Around 83% of women with vasa praevia have at least one identifiable risk factor. The most common ultrasound finding in cases of vasa praevia is a velamentous or marginal cord insertion into the placenta.

Certain findings should raise the index of suspicion:

• Velamentous cord insertion
• Bilobed or succenturiate-lobed placenta
• A low-lying placenta or placenta praevia in the second trimester
• Pregnancies conceived through IVF or other assisted reproductive technology
• Multiple pregnancy

Risk factors are common, and most women with them will not have vasa praevia. For example, velamentous cord insertion occurs in roughly 2% of pregnancies, while vasa praevia affects under 1%. Risk factors raise the index of suspicion and determine who needs a follow-up scan – they are not the diagnosis itself.

How is vasa praevia diagnosed?

A diagnosis of vasa praevia is made with ultrasound. Fetal vessels within 20 mm of the internal cervical os are diagnostic of vasa praevia. Vessels between 20 mm and 50 mm from the internal cervical os are considered low-lying fetal vessels. International guidelines recommend that the optimal assessment for vasa praevia should be performed with a transvaginal ultrasound at the time of the morphology scan (20-22 weeks) (ISUOG, 2020). Transvaginal ultrasound provides high resolution of small vessels and a more accurate assessment of the vessel-to-os distance.

RANZCOG suggests offering screening to all women. A good quality report will comment on the location of the placental cord insertion, the morphology of the placenta, and clear documentation of a colour Doppler sweep of the cervix to assess for low-lying fetal vessels.

A follow-up transvaginal scan at 28–32 weeks should be arranged if any risk factors are identified at the morphology scan or if a woman has not had a prior scan.

Where vessels are within 50 mm at the morphology scan, a repeat scan at or after 32 weeks is reasonable to confirm their final location.

Transvaginal colour Doppler image showing fetal vessels overlying the internal cervical os (the opening of the cervix)

When to refer to a tertiary unit

For GPs and clinicians in shared-care or community settings, the referral triggers are straightforward:

• A suspected or confirmed finding on ultrasound should prompt referral to specialist obstetric care for follow-up scans at 28 and 32 weeks, and ongoing management planning.
• Suspected vasa praevia, together with clinical features of preterm labour or antepartum haemorrhage occurring before the third-trimester confirmatory scan, warrants consultation with a Maternal Fetal Medicine (MFM) specialist for management of the pregnancy.
• At the extremes of prematurity, or where the diagnosis is uncertain, MFM consultation is again advisable to balance the competing risks.

An equivocal or incomplete view of the cord insertion or the vessels near the os is reasonable grounds to refer for a closer look. The aim is simply to ensure that, where the risk exists, the vessels have been specifically assessed. When vasa praevia is excluded, that is genuine reassurance for the patient and for you. When it is identified, a clear plan can be put in place well before it matters.

Delivery and management

Once vasa praevia is confirmed, patient management centres on planning a caesarean birth before the onset of labour, while weighing the risks of prematurity against the risk of vessel rupture. Care should be individualised and discussed in the context of patient preferences and individual circumstances.

For asymptomatic women with confirmed vasa praevia, inpatient versus outpatient management from 32+0 weeks should be discussed rather than mandated. Outpatient management is reasonable for asymptomatic women, particularly those without additional risk factors and with good access to emergency services. When weighing this up, consider additional risk factors for preterm labour – multiple gestation, a short cervix, or a history of preterm birth. Women with low-lying fetal vessels (20–50mm) can generally be managed as outpatients with ongoing antenatal care and birth planning.

Current guidance, balancing prematurity against the risk of vessel rupture in labour:

• Asymptomatic, no preterm-birth risk factors: consider caesarean from 36 +0 to 36 +6. Prolonging to around 36 weeks appears to best balance the risks of prematurity and of vasa praevia complications.
• Symptomatic (bleeding, preterm labour): consider urgent caesarean, balancing the risks of prematurity against the likelihood of fetal compromise

Refer your patient to QUFW

At QUFW, our consultants are subspecialist obstetricians and gynaecologists with extensive experience with high-risk pregnancies. Vasa praevia is uncommon, but it is important that primary care providers offer a routine ultrasound at 20-22 weeks (morphology scan) to assess for this condition in all pregnancies.

If you would like to contact us to discuss a patient you would like assessed, or if your patient would like to book an appointment, click here.

Sources: This article draws on the Royal Australian and New Zealand College of Obstetricians and Gynaecologists (RANZCOG) patient information on vasa praevia. For more, see the RANZCOG resource hub at ranzcog.edu.au/resource-hub.

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