Definition and incidence
• Fetal vessels coursing through the membranes over the internal cervical os and below the
fetal presenting part, unprotected by placental tissue or the umbilical cord.
• This can be secondary to a velamentous cord insertion in a single or bilobed placenta (vasa
praevia type 1), or from fetal vessels running between lobes of a placenta with one or more
accessory lobes (vasa praevia type 2).
• Incidence – 1 in 2000 to 6000 pregnancies, but the condition may be under-reported.

Risks or complications
• When the fetal membranes are ruptured (spontaneously or artificially), the unprotected
fetal vessels are at risk of disruption with consequent fetal haemorrhage.
• Because the fetal blood volume is around 80–100 ml/kg, the loss of relatively small
amounts of blood can have major implications for the fetus.
• The mortality rate is around 60%, although significantly improved survival rates of up to
97% have been reported where the diagnosis has been made antenatally.

Risk factors
• Placental anomalies such as a bilobed placenta or succenturiate lobes where the fetal vessels
run through the membranes joining the separate lobes together.
• History of low-lying placenta in the second trimester.
• Multiple pregnancy and IVF where the incidence of vasa praevia has been reported to be as
high as 1 in 300. The reasons for this association are not clear, but disturbed orientation of
the blastocyst at implantation, vanishing embryos, and the increased frequency of placental
morphological variations have been postulated.
• The incidence of velamentous cord insertion in an unselected population is around 1%,
and that of bilobed or succenturiate placenta is around 1.7%, two-thirds of which have a
velamentous cord insertion. The reported coexistence of velamentous cord insertion and
vasa praevia has been reported to be between 2 and 6%. Adding the other groups thought
to be at increased risk of vasa praevia, i.e., multiple pregnancies, IVF conceptions, and
at increased risk of vasa praevia.

Screening
• Although vasa praevia is detectable by USS, there remains insufficient information on
the case definition, natural history, and epidemiology of the condition. The accuracy and
practical application of the screening test has not been elucidated in the general pregnant
population. There is currently no agreed management pathway for those with confirmed
vasa praevia. Taking all of this into account, there is uncertainty about the balance of
benefit versus harm to be derived from screening all pregnant women with a view to
offering CS to those at risk.

Clinical features :
In the absence of vaginal bleeding:
• Antenatal period – there is no method to diagnose vasa praevia clinically.
• Intrapartum period – vasa praevia can occasionally be diagnosed by palpation of fetal
vessels in the membranes at the time of vaginal examination. However, high index of
suspicion is necessary if one feels something unusual and to confirm the diagnosis prior to
membrane rupture.
• Direct visualization using an amnioscope has some use, but this only gives visual access to
the area of membranes exposed by the dilated cervix.
• Very rarely, FHR abnormalities in the absence of bleeding may be present secondary to
compression of the fetal vessels by the fetal presenting part.
In the presence of vaginal bleeding:
• Vasa praevia often presents with fresh vaginal bleeding at the time of membrane rupture
and FHR abnormalities. More rarely, bleeding can occur in the absence of membrane
rupture.

Tests to differentiate between fetal and maternal bleeding
• Various tests exist that can differentiate between fetal and maternal blood, but they are
often not applicable in the clinical situation.
• The Kleihauer–Bekte test and haemoglobin electrophoresis accurately identify fetal cells
in the maternal circulation and fetal haemoglobin. Both can detect the presence of fetal
haemoglobin in concentrations as low as 0.01%, and both can be used to identify fetal
cells in vaginal blood loss. The disadvantage – they are laboratory-based tests that take a
significant amount of time before a result is obtained, thus rendering them of little use in
this clinical situation.
• The resistance of fetal haemoglobin to denaturation with alkali has been used by various
methods to identify fetal bleeding. Lindqvist and Gren described a simpler bedside test
using 0.14 M sodium hydroxide solution, which denatures adult haemoglobin, turning it a
brownish-green colour, while fetal haemoglobin is resistant to denaturation and retains its
red colour. This method may have some applicability in the clinical situation but requires
further validation.

Ultrasound scan
• Vasa praevia can be accurately diagnosed with colour Doppler TVS. The ultrasound appearance of vasa
praevia is of linear echolucent structures overlying the cervix. The use of colour Doppler improves the
diagnostic accuracy.
• Factors such as maternal obesity, scarring, and fetal position can influence accuracy, and care must be taken
not to mistake cord presentation for vasa praevia. Using both the abdominal and vaginal routes of scanning
and changing maternal position can improve diagnostic accuracy.
• In cases with the diagnosis made antenatally, the perinatal outcome is significantly better than those
diagnosed acutely.

Management

Suspected vasa praevia
• Transvaginal colour Doppler USS to confirm the diagnosis.
• If there is either an USS or clinical suspicion of vasa praevia in the absence of
fetal compromise, undertake a formal systematic assessment of the region of the
internal cervical os by transvaginal colour Doppler USS.

Bleeding vasa praevia
• Deliver by category 1 emergency CS – if signs of acute fetal compromise are
present, to minimize the risk of fetal exsanguination.
• Delay to facilitate USS or transfer to another unit could result in fetal demise.
• Because of the speed at which fetal exsanguination can occur and the high
perinatal mortality rate associated with ruptured vasa praevia, do not delay
delivery trying to confirm the diagnosis if fetal wellbeing is compromised.

Vasa praevia identified in the second trimester
• Repeat imaging in the third trimester to confirm persistence. As gestation
advances vasa praevia can resolve in up to 15% of cases. To avoid unnecessary
anxiety, admissions, prematurity, and CS, it is essential to confirm persistence of
vasa praevia in the third trimester.

Confirmed vasa praevia at term
• Deliver by elective CS – in view of the risk of fetal haemorrhage with the onset
of labour or membrane rupture and the minimal risks of neonatal lung disease,
once vasa praevia has been confirmed at term, deliver by elective CS as soon as
is practicable.

 

Confirmed vasa praevia in the third trimester
• Antenatal admission from 28 to 32 weeks of gestation to a unit with appropriate neonatal facilities
will facilitate quicker intervention in the event of bleeding or labour.
• In view of the increased risk of PTD, consider corticosteroids for fetal lung maturity.
• Outpatient management may be possible if there is no evidence of cervical shortening on TVS and
there are no symptoms of bleeding or preterm uterine activity.
• Laser ablation in utero may have a role in the treatment of vasa praevia.

Deliver by elective CS prior to the onset of labour, between 35 and 37 weeks of
gestation, when the risks of prematurity have significantly decreased.