Vasa praevia

Definition:

Vasa praevia is an obstetric condition where fetal blood vessels cross or run near the internal os of the cervix beneath the fetal presenting part, unsupported by the placenta or umbilical cord. These vessels are part of the fetal circulation, connecting the placenta to the fetus, and are at risk of rupture when membranes rupture, leading to fetal exsanguination and death if not promptly diagnosed and managed.

Epidemiology:

Vasa praevia is rare, occurring in approximately 1 in 2,500 to 5,000 pregnancies.
The condition is associated with a high risk of fetal mortality if undiagnosed before labor begins.

Risk Factors:

Velamentous cord insertion (the umbilical cord inserts into the fetal membranes outside the placental margin, then travels within the membranes to the placenta).
Succenturiate-lobed placenta (a placenta with one or more accessory lobes connected to the main placenta by blood vessels).
In vitro fertilization (IVF) pregnancies.
Multiple gestations.

Pathophysiology:

In vasa praevia, the fetal blood vessels, unprotected by Wharton’s jelly and the placental tissue, traverse the fetal membranes across the lower segment of the uterus. These vessels are vulnerable to rupture when the membranes rupture or are artificially ruptured during labor, leading to rapid fetal hemorrhage and potential death due to fetal exsanguination.

Clinical Features

Signs and Symptoms:

Painless vaginal bleeding in the second or third trimester, particularly after membrane rupture.
Fetal heart rate abnormalities, such as bradycardia, following membrane rupture, indicating fetal distress.

Diagnosis:

Prenatal Ultrasound: Color Doppler ultrasound is the diagnostic modality of choice for detecting vasa praevia, ideally performed in the second trimester or early third trimester.
Transvaginal Ultrasound with Color Doppler: Provides high sensitivity in diagnosing vasa praevia by visualizing fetal blood vessels crossing the internal cervical os.

Management

Antenatal Management:

Once diagnosed, the pregnancy is considered high risk. Expectant management involves close monitoring and planning for delivery in a controlled setting.
Corticosteroids may be administered to accelerate fetal lung maturity if early delivery is anticipated.
Hospitalization may be recommended for close monitoring if there’s evidence of vasa praevia bleeding or if the pregnancy is approaching term.

Delivery Planning:

Planned preterm cesarean delivery, typically between 34-37 weeks gestation, before the onset of labor or membrane rupture, to prevent vessel rupture and fetal exsanguination.
The exact timing of delivery should be individualized based on the presence of bleeding, fetal lung maturity, and other obstetric considerations.

Prevention and Prognosis:

Early and accurate diagnosis through targeted ultrasound examination is crucial for preventing fetal mortality associated with vasa praevia.
With timely diagnosis and management, the prognosis for fetuses with vasa praevia has significantly improved, with survival rates approaching 97% in cases where prenatal diagnosis is made.

Summary

Vasa praevia is a critical yet rare obstetric condition characterized by the presence of fetal blood vessels across the internal cervical os, putting the fetus at risk for life-threatening hemorrhage upon membrane rupture. Early detection through color Doppler ultrasound, careful monitoring, and planned cesarean delivery before labor onset are key to managing this condition and preventing fetal mortality. Understanding the risk factors, pathophysiology, clinical presentation, and management strategies for vasa praevia is essential for medical students and healthcare providers involved in prenatal care and obstetrics.