Vasa praevia: ultrasound diagnosis at the mid-trimester scan.

: Vasa praevia is a condition in which fetal vessels, unsupported by the umbilical cord or placenta, run on the fetal membranes in the lower uterine segment near the cervix. Rupture of these vessels prior to or at the time of delivery is associated with a very high perinatal mortality rate. Antenatal diagnosis of this condition with ultrasound is readily available, alters management and significantly decreases the risk of fetal loss. : We describe our experience of diagnosing vasa praevia at a tertiary referral ultrasound centre. The ultrasound characteristics that led to the diagnosis of vasa praevia and associated risk factors will be discussed. A guide to aid sonographers when performing an obstetric examination to help screen for this condition has been included. : Where suspicion of vasa praevia is raised, and cannot be ruled out on transvaginal ultrasound, a third trimester scan is required.