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.
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 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.
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.
As mentioned earlier, some conditions increase the risk of vasa praevia. These include:
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.
It is difficult to envisage a situation where all cases of vasa praevia will be detected prior to labour onset. In many case, probably most, there is no real risk factor identifiable, to trigger a prenatal look out for these vessels.
When the bleeding occurs when the labouring woman is already in hospital, an emergency caesarean section will be done and this will, in almost all cases, save the baby. However, if the woman is still at home, the prognosis for the baby is very poor indeed. Mortality rates (for babies) for undiagnosed vasa praevia presenting away from hospital are estimated to be as high as 90%.
Aberrant vessels of vasa praevia
Internal opening of the cervix