Vasa Previa: Screening and Transvaginal Doppler Criteria
Definition
According to ISUOG recommendations (Practice Guidelines: performance of third-trimester obstetric ultrasound scan, 2024), vasa previa is characterized by demonstration on transvaginal ultrasound with color Doppler imaging of cord insertion into the membranes over the cervix, from which unprotected vessels cross toward the placenta. The criterion is also defined as unprotected fetal vessels crossing the membranes over the internal cervical os or passing within <20 mm from the internal os.
Screening in the Presence of Risk Factors
According to ISUOG, in the presence of vasa previa risk factors, targeted transvaginal examination is recommended—depending on experience and available resources. Initially, this recommendation applies to routine second-trimester screening (mid-trimester scan), and the same recommendation extends to third-trimester examination.
The specific list of risk factors is not provided in the available fragments—[to be clarified].
Transvaginal Doppler Methodology
The foundation is transvaginal access with application of color Doppler mapping. When interpreting color mapping, the following general methodological principles should be kept in mind:
- color encodes flow direction relative to the transducer, not vessel type (artery/vein);
- excessive gain results in blooming artifact affecting the entire field;
- insonation angle close to 90° may produce false "occlusion"—loss of signal.
Assessment of Distance from the Internal Cervical Os
| Criterion | Value |
|---|---|
| Unprotected vessels over the internal cervical os | Vasa previa |
| Vessels at distance from the internal os | <20 mm — vasa previa (ISUOG, 2024) |
Additionally, the literature describes the possibility of three-dimensional ultrasound diagnosis of vasa previa (3D ultrasonography), as well as classification of vasa previa types (in particular, type II) in the context of in utero treatment.
Frequently asked questions
What is the standard approach for diagnosis of vasa previa?
Transvaginal ultrasound with color Doppler imaging, demonstrating cord insertion into the membranes over the cervix and unprotected vessels crossing toward the placenta (ISUOG, 2024).
What distance from the internal cervical os defines the criterion?
Unprotected fetal vessels crossing the membranes over the internal cervical os or passing within <20 mm from the internal os (ISUOG, 2024).
When is targeted transvaginal examination indicated?
In the presence of vasa previa risk factors, targeted transvaginal examination is recommended, depending on experience and available resources; the recommendation applies to screening in both the second and third trimesters (ISUOG, 2024).
Can 3D ultrasound be used in vasa previa diagnosis?
The literature describes prenatal diagnosis of vasa previa using three-dimensional ultrasound sonography (3D ultrasonography).