Vasa Previa: Prenatal Diagnosis and the Rationale Behind Using a 5 cm Distance from Internal Os

In pregnancies with vasa previa, prenatal diagnosis and pre-labor cesarean delivery are associated with significantly improved perinatal outcomes compared to undetected cases. However, a universally accepted ultrasonographic definition of vasa previa is lacking. Specifically, the distance from the cervical internal os beyond which vaginal delivery can be safely recommended remains to be determined. Field experts and recently published societal guidelines agree that a 2 cm cut-off is suboptimal, given that complete cervical dilation during labor risks unprotected fetal vessels within a 5 cm radius from the internal os. Thus, in the setting of a scarcity of evidence and case reports of perinatal death with unprotected fetal vessels beyond 2 cm from the internal os, a more conservative definition that includes unprotected fetal vessels located within 5 cm of the internal os is imperative to improve outcomes.