Vasa previa are umbilical vessels which run along the fetal membranes over or near the internal cervical os. Unprotected by the umbilical cord or placenta these vessels most often originate from velamentous insertion, low-lying and/or bilobated placenta. Ante- or intrapartum rupture of fetal membranes can provoke laceration of vasa previa causing painless vaginal bleeding with rapid fetal exsanguination and demise. Reduction of associated perinatal morbidity and mortality depends entirely on accurate antenatal diagnosis. In this review we present two cases of prenatally diagnosed vasa previa with successful perinatal outcome. An outline of the major sonographic markers and criteria used in the ultrasound diagnosis and differential diagnosis, as well as a step-by-step algorithm of the ultrasound examination in cases with suspected vasa previa is proposed.