Thank you for your letter and interest in the SMFM Consult Series number 37, Diagnosis and Management of Vasa Previa, with respect to the following issues: definition of vasa previa, ultrasound diagnosis, and ultrasound imaging.

With respect to definition of vasa previa, the introduction of the Consult Series states that “vasa previa occurs when fetal blood vessels that are unprotected by the umbilical cord or placenta run through the amniotic membranes and traverse the cervix” (see What is vasa previa?). This definition includes both arterial and venous vessels.

With respect to the question regarding ultrasound diagnosis of vasa previa, the Consult Series states, “Although there are no standardized criteria for how close the fetal vessels must be to the internal os to constitute vasa previa, a threshold of 2 cm has been proposed. In 1 series, all emergent deliveries with vasa previa had a fetal vessel within 2 cm of the cervical os.”

In addition it states that “routine ultrasound evaluation of the placenta and lower uterine segment permits detection of the majority of cases. In a recent systematic review of 8 series that included > 400,000 pregnancies and 138 cases of vasa previa, the median detection rate was 93%, with a specificity of 99%. Although it can be diagnosed antenatally by transvaginal ultrasound scanning, vasa previa can be missed even under optimal circumstances.” We believe these statements are accurate and adequately evidence based.

With regard to the final comment regarding the quality of the imaging for vasa previa, we appreciate the image submitted by Drs Catnanzarite and Oyelese and agree they nicely demonstrate several key landmarks.