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Vasa previa: causes, risk factors, symptoms, diagnosis, treatment and risks for the foetus and mother

By Cristiano Antonino On Jan 10, 2023

Vasa praevia (or ‘vasa previa’ or ‘vasa previ’) are an obstetrical complication characterised by the presence of fetal blood vessels that run near or in front of the internal orifice of the uterus

These vessels, located within the membranes of the yolk sac but without the support of the umbilical cord or placenta, are at risk of bleeding when the rupture of the supporting membranes of the pregnancy occurs.

Vasa previa and placenta previa

Vasa previa could be considered a type of placenta previa, however, the two conditions are distinct.

To simplify the concepts:

in vasa previa the blood vessels carrying nourishment to the foetus are located in front of or near the cervix;
on the contrary in placenta previa (or ‘low placenta’) it is the placenta itself that is placed in front of or near the cervix.

In both cases, the risks are high for both the woman and the foetus.

The expression ‘vasa previa’ is derived from Latin; ‘vasa’ means vessels and ‘previa’ comes from ‘pre’ meaning ‘before’ and ‘via’ meaning ‘away’, indicating that the vessels are located before the foetus in the birth canal.

This condition occurs in about 6 in 10000 pregnancies.

Causes of vasa previa

Vasa praevia occur when unprotected fetal vessels pass through the fetal membranes near or above the cervix, through which the fetus makes its way to the vagina during delivery.

These vessels may originate from a velamentous insertion of the umbilical cord or they may join an accessory placental lobe (succenturiate) to the main placental disc.

If these fetal vessels rupture, haemorrhage comes from the fetoplacental circulation and bleeding of the fetus will occur rapidly, possibly resulting in its death.

Vasa previa is thought to result from an early placenta previa

As pregnancy progresses, the placental tissue surrounding the vessels above the cervix undergoes atrophy and the placenta grows preferentially towards the upper part of the uterus: this leaves unprotected vessels flowing over the cervix and into the lower uterine segment, as demonstrated using serial ultrasound.

Olese et al. found that two-thirds of patients with vasa previa at delivery had low placenta or placenta previa that resolved before delivery.

Risk factors include low placenta and in vitro fertilisation

This condition is more easily observed in cases of velamentous insertion of the umbilical cord, in the presence of accessory placental lobes and in multiple pregnancies.

Diagnosis of vasa previa

The classic clinical triad consists of ruptured membrane, painless vaginal bleeding and fetal bradycardia.

It is a condition that can seldom be confirmed prior to delivery, but can be suspected by observing on echocolordoppler a blood flow through the internal uterine orifice.

The diagnosis is usually confirmed after delivery by testing the placenta and fetal membranes, and often by the time the diagnosis is made the fetus is already dead, as blood loss constitutes a significant part of the fetal blood volume.

Types of vasa previa

There are three types of vasa previa. Types 1 and 2 were described by Catanzarite et al:

In type 1, there is a velamentous insertion with vessels running above the cervix.
In type 2, unprotected vessels flow between the lobes of a bilobed lobate or succenturiate placenta.
In type 3, a portion of the placenta overlying the cervix undergoes atrophy. In this type, there is a normal insertion of the placental cord and the placenta has only one lobe, but the vessels at the edges of the placenta are exposed.

In the case of vasa previa, the main risk is exsanguination of the foetus, which often leads to death.

Treatment

It is recommended that women with vasa previa deliver by elective caesarean section before the membranes rupture.

Since the time of rupture of the membrane is difficult to predict, elective caesarean section is recommended at 35-36 weeks.

This gestational age provides a reasonable balance between the risk of death and prematurity.

Since these patients are at risk of preterm delivery, steroids are recommended to promote foetal lung maturation.

When haemorrhage occurs, the patient goes into labour or, if the membranes rupture, immediate treatment with an emergency caesarean section is usually indicated.

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