Vasa praevia: ultrasound diagnosis at the mid‐trimester scan

Abstract

Background: 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. Materials and methods: 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. Conclusions: Where suspicion of vasa praevia is raised, and cannot be ruled out on transvaginal ultrasound, a third trimester scan is required.

Detection of vasa praevia in the mid‐trimester ultrasound by Australian sonographers

“…The systematic review by Ruiter et al demonstrated 83% of vasa praevia cases had one or more clinical risk factors, highlighting that occasionally, a vasa praevia case will have no detectable risk factors. Screening every woman in the mid‐trimester ultrasound transabdominally over the lower uterine segment with colour Doppler should improve the detection in these cases (Figures and ) …”

“…Another risk factor for vasa praevia is a succenturiate lobe placenta, this is due to the unsupported communicating vessels between the two placental components to course over the cervical IOS . It has been reported that if a woman has a succenturiate lobe placenta, the risk of vasa praevia is as high as 1 in every 37 cases .…”

“…The third risk factor for vasa praevia is a low‐lying placenta because of the potential for placental vessels to remain in the lower uterine segment after placental migration and trophotropism in the third trimester (Figure and ) . A low‐lying placenta in the mid‐trimester ultrasound is defined as the lower placental edge being 20 mm or less from the cervical IOS.…”

“…A transvaginal ultrasound is already a well‐established standard of care to assess the cervical length and assessing for vasa praevia transvaginally has identical patient preparation and imaging technique. The only difference is the colour box is placed over the lower uterine segment and the transducer is swept longitudinally and transversely identifying fetal vessels within 20 mm of the cervical IOS (Figures , , , and ). RANZCOG recommendation 2 identifies colour and pulse wave Doppler are the most accurate means to diagnose vasa praevia .…”

“…Vasa praevia is a silent condition with potentially fatal consequences for the fetus. It is defined as fetal vessels (unsupported by either the Wharton's jelly in the umbilical cord or placental tissue) that run through the amniotic membranes in close proximity to or overlying the cervix . The estimated incidence of vasa praevia is 1:1250 – 1:4667 pregnancies .…”