[Prenatal diagnosis of vasa previa: A case report]ResumenLa vasa previa es una entidad patologica considerada como causa de sangrado en la segunda mitad del embarazo que generalmente no es diagnosticada debido a que no se busca de rutina. Caso: Mujer de 30 anos, cursando su tercer embarazo. En la semana 28 1/7 se realiza control ecografico y se diagnostica vasa previa. A las 35 1/7 semanas acude con historia de sangrado transvaginal activo sin modificaciones cervicales y frecuencia cardiaca fetal en limite superior, por lo que se indica cesarea de emergencia. Nace bebe masculino, presentacion pelvica, 2520g, Apgar 8/9. Patologia reporta lobulo accesorio placentario con estructura vascular que surge de la placenta principal. Conclusiones: La vasa previa es una patologia causal de sangrados de la segunda mitad, poco frecuente segun las estadisticas, sin embargo, esto puede ser secundario al subregistro derivado de la falta de tamizaje. El diagnostico prenatal mejoras los resultados perinatales. AbstractVasa previa is a pathological entity considered as a cause of bleeding in the second half of pregnancy that is generally not diagnosed because it is not routine to search for it. Case: 30 year old woman, during her third pregnancy. A 28 1/7 week ultrasound is performed and a vasa previa is diagnosed. At 35 1/7 weeks she appeared with active transvaginal bleeding without cervical modifications and fetal heart rate in upper limit, so emergency caesarean section is performed. A male baby is born, pelvic presentation, 2520g, Apgar 8/9. Pathology reports placental accessory lobe with a vascular structure arising from the main placenta. Conclusions: Previous vasa is a causal pathology of bleeding in the second half, infrequent according to statistics, however, this may be secondary to under-recording due to lack of screening. Prenatal diagnosis improves perinatal outcomes.**