Risk Factors for Vasa Previa
The most significant risk factors for vasa previa include velamentous cord insertion, placenta previa, multiple gestations, and pregnancies conceived through in vitro fertilization. 1, 2
Primary Risk Factors
- Velamentous cord insertion: When the umbilical cord inserts into the membranes rather than directly into the placenta, creating unprotected fetal vessels that can traverse the cervix 3
- Second-trimester placenta previa or low-lying placenta: Particularly those that later resolve, leaving vessels running across the lower uterine segment 2, 4
- Multiple gestations: Twin pregnancies have a higher frequency of velamentous cord insertion and consequently vasa previa 3
- In vitro fertilization (IVF): Pregnancies conceived through assisted reproductive technologies have increased risk 2, 5
Additional Risk Factors
Placental abnormalities:
Prior uterine surgery: Including cesarean delivery and other procedures that may affect placentation 3
Clinical Significance
Vasa previa is a rare but potentially catastrophic condition where fetal blood vessels, unsupported by either the umbilical cord or placental tissue, traverse the fetal membranes over the cervical os 3, 1. The condition carries a high risk of fetal mortality due to potential vessel rupture and rapid fetal exsanguination when membranes rupture 5, 6.
Screening Recommendations
- Targeted screening: Patients with identified risk factors should undergo transvaginal ultrasound with color Doppler to evaluate for vasa previa 2, 4
- Cord insertion assessment: Evaluation of placental cord insertion site during the 20-week anatomy scan can help identify cases at risk 4
- Multiple gestation evaluation: Special attention should be paid to the placenta, umbilical cords, and cervix during anatomical survey of multiple gestations 3
Management Implications
When vasa previa is diagnosed prenatally:
- Planned cesarean delivery before labor or membrane rupture is recommended 1, 6
- Delivery timing is typically between 34-36 weeks gestation after administration of corticosteroids 2, 6
- Hospitalization may be considered from 30-34 weeks gestation for close monitoring 6
Diagnostic Pitfalls to Avoid
- Failure to identify risk factors during prenatal care 5
- Inadequate imaging techniques (transvaginal ultrasound with color Doppler is superior to transabdominal ultrasound alone) 5, 4
- Digital pelvic examination without first excluding vasa previa can trigger vessel rupture 7
Early identification of risk factors and appropriate screening can dramatically improve outcomes, with recent studies showing excellent survival rates when vasa previa is diagnosed prenatally compared to dismal outcomes when undiagnosed before labor 1, 6, 4.