Type Three Vasa Previa Diagnosed in Late Pregnancy Despite Apparent Placental Migration: A Case Report

Type three vasa previa is a rare and potentially fatal obstetric condition in which unprotected fetal blood vessels traverse the cervical membranes near the internal os despite normal placental morphology and umbilical cord insertion. Owing to an absence of classical risk factors, the prenatal diagnosis of vasa previa is particularly challenging; therefore, the condition may go undiagnosed, especially after apparent placental migration. A 36-year-old multiparous woman was referred to our clinic at 33 weeks and three days of gestation with suspicions of a low-lying placenta. Follow-up sonography performed at 35 weeks and two days of gestation revealed an apparent placental migration to the normal position; however, a detailed transvaginal color Doppler sonogram incidentally revealed a pulsatile fetal blood vessel crossing the internal os. The patient was diagnosed with type three vasa previa and underwent an elective cesarean delivery at 35 weeks and three days of gestation, resulting in the birth of a healthy male infant. A postnatal examination of the placenta and umbilical cord confirmed the presence of a membrane-traversing fetal blood vessel in the absence of a velamentous cord insertion or accessory placental lobes. This case highlights a critical diagnostic pitfall, and the apparent resolution of placental malposition does not exclude the presence of type three vasa previa. Therefore, continued third-trimester surveillance with transvaginal color Doppler sonography is essential to avoid missed diagnoses and prevent a potentially catastrophic fetal hemorrhage.