Vasa Previa

What happens in vasa previa

Normally, the umbilical cord inserts into the middle of the placenta, and the baby's blood vessels are protected inside the cord's jelly-like tissue. In vasa previa, some fetal vessels run unprotected through the membranes (the amniotic sac) and pass over or within about 2 cm of the cervix.

When the membranes rupture or the cervix dilates, these vessels can tear. A baby at term has only about 80 to 100 ml of blood per kilogram, so even a small bleed can cause fetal shock and death within minutes. Vessels can also be compressed by the baby's head, reducing blood flow.

Types

• Type 1: associated with velamentous cord insertion, where the cord inserts into the membranes instead of the placenta, and vessels travel through the membranes to the placenta
• Type 2: vessels run between two lobes of a bilobed or succenturiate (extra-lobed) placenta
• Type 3: a variant where vessels run from the edge of the placenta over the cervix

Risk factors

• low-lying placenta or placenta previa in the second trimester, even if it later moves up
• velamentous cord insertion
• bilobed or succenturiate placenta
• IVF and other assisted reproduction
• multiple pregnancy

Symptoms

Vasa previa usually causes no symptoms before labor. Possible signs:

• painless vaginal bleeding, especially when the waters break
• changes in the baby's heart rate during monitoring
• reduced fetal movement

Because symptoms appear only when it is already an emergency, diagnosis by ultrasound before labor is crucial.

Diagnosis

• anatomy ultrasound at around 18 to 22 weeks: checks where the cord inserts into the placenta and whether the placenta is low-lying
• transvaginal ultrasound with color Doppler: the key test, showing vessels crossing over or near the cervix
• repeat scans in the third trimester; about 1 in 5 cases diagnosed in the second trimester resolve

Screening for vasa previa is recommended when risk factors are present, such as a low-lying placenta, velamentous cord insertion, a bilobed placenta or IVF.

Management

• regular ultrasound monitoring
• avoid sex and vaginal exams after diagnosis, as advised
• hospital admission may be offered from about 30 to 34 weeks, close to an operating room, especially with a short cervix or bleeding
• corticosteroids to mature the baby's lungs before planned early delivery
• planned cesarean delivery at about 34 to 37 weeks, before labor begins
• neonatal team on standby at delivery for possible blood transfusion

Emergency delivery

If bleeding starts or waters break, an emergency cesarean is needed immediately. The newborn may need urgent blood transfusion.

Outlook

When vasa previa is diagnosed before labor and managed with planned cesarean, survival is over 95 percent. When undiagnosed, fetal death rates are very high. Babies born early may need neonatal care for prematurity.

Frequently asked questions

What is vasa previa?

It is a condition where the baby's blood vessels run unprotected through the membranes over or near the cervix. If the membranes rupture, these vessels can tear and the baby can lose blood quickly. It is rare but very dangerous if not diagnosed.

How is vasa previa diagnosed?

It is diagnosed with transvaginal ultrasound and color Doppler, which shows blood vessels crossing the cervix. It is usually looked for at the mid-pregnancy anatomy scan, especially when risk factors such as a low-lying placenta or IVF are present.

Can you have a vaginal birth with vasa previa?

No. Vaginal birth is not recommended because labor and rupture of the membranes can tear the fetal vessels. A planned cesarean around 34 to 37 weeks is the standard approach.

What are the risk factors for vasa previa?

Risk factors include a low-lying placenta or placenta previa, velamentous cord insertion, bilobed or succenturiate placenta, IVF pregnancies and multiple pregnancies.

Is vasa previa the same as placenta previa?

No. In placenta previa, the placenta itself covers the cervix, and bleeding is mostly maternal. In vasa previa, the baby's blood vessels cross the cervix, and bleeding is from the baby. They can occur together.