Vasa Praevia

What is Vasa Praevia?

This is a situation where blood vessels (or their branches) in the umbilical cord develop a route away from the cord and placenta proper and are found just above the cervical opening. That means they are, in effect, between the baby’s leading part (usually the head) and the cervical opening.

How does vasa praevia present?

There is usually no problem with this vessel position until labour starts. Once the cervix starts dilating, the delicate vessels will be stretched and will soon tear. This will unleash torrential vaginal bleeding. This is actually the baby’s blood, which makes it rapidly fatal for the unborn baby.

Sometimes the bleeding will occur before actual labour onset, when the waters (membranes) break. Since the vessels are running in the membranes, this will cause the aberrant vessels to rupture and unleash bleeding. This might also occur when membrane are ruptured artificially to induce labour.

Sometimes there may not be any bleeding. Instead, there will be fetal heart rate pattern abnormalities as the unruptured vessels are compressed by the descending head in the pelvis, typically in very early stages of labour. This will normally trigger intervention.

How common is vasa praevia?

The exact incidence is not known but estimates range from 1 in 2000 – 3000 deliveries. Some situations increase the likelihood of vasa praevia as we shall see shortly.

What are the predisposing factors for vasa praevia?

As mentioned earlier, some conditions increase the risk of vasa praevia. These include:

A low lying placenta (placenta praevia)
Multiple pregnancy (twins or more)
A placenta with an extra lobe (succenturiate lobe)
A placenta where the cord insertion is peripheral rather than central (velamentous insertion of cord)
IVF pregnancy: For some obscure reason, IVF pregnancy increases the risk of vasa praevia almost 10-fold to an estimated 1 in 300.
Previous history of uterine surgery including minor procedures such as D&C

How is vasa praevia diagnosed?

Vasa praevia is not routinely looked for on a routine detailed anatomy scan usually done at around 20 weeks. However, the person performing this scan will look for this possible diagnosis if the woman has one of the risk factors mentioned above.

It is important to be cautious however: A scan performed at 20 weeks will quite possibly miss up to two-thirds (66%) of all cases of vasa praevia. Moreover; since this routine scan is usually abdominal, it makes it more difficult to see these vessels, which in any case, are still relatively small at this stage. A trans-vaginal scan is much better at detecting these vessels.

For those women found to have a low-lying placenta, it is common practise to repeat the scan later in the third trimester, around 34 weeks gestation. This is done to rule out placenta praevia. A look out for aberrant vessels of vasa praevia is invariably done at this scan.

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vasa praevia can cause severe bleeding which could be fatal for the baby

Umbilical Cord

Aberrant vessels of vasa praevia

Internal opening of the cervix

Placenta