A velamentous umbilical cord is characterized by membranous umbilical vessels at the placental insertion site; the remainder of the cord is usually normal. Membranous vessels can also arise as aberrant branches of a marginally inserted umbilical cord or they can connect lobes of a bilobed placenta or the placenta and a succenturiate lobe. Because of the lack of protection from Wharton’s jelly, these vessels are prone to compression and rupture, especially when they are located in the membranes covering the cervical os (ie, vasa previa).
VELAMENTOUS UMBILICAL CORD
Definition — In a velamentous umbilical cord insertion, the placental end of the cord consists of divergent umbilical vessels surrounded only by fetal membranes, with no Wharton's jelly. The length of the membranous vessels, ie, the distance between the end of the normal cord and the placental insertion, is highly variable.
Prevalence — Velamentous insertion occurs in approximately 1 percent of singleton gestations , but is observed in as many as 15 percent of monochorionic twin gestations [2-4]. It is also more common in placenta previa than in normally located placentas. The prevalence may be slightly higher in stillbirths, particularly from multifetal pregnancies .
Pathogenesis — The pathogenesis of velamentous cord insertion is unknown. The most popular hypothesis is that the cord is initially inserted centrally, but its location progressively becomes peripheral as one half of the placenta actively proliferates toward the well-vascularized uterine fundus (trophotropism) while the other pole involutes; the umbilical cord is unable to follow the migration of the placenta . The association of velamentous cord insertion and placenta previa supports this hypothesis.
Ultrasound and gross examination — On ultrasound and gross examination, the normal umbilical cord sheath is contiguous with the chorionic plate. With a velamentous insertion, the cord can end several centimeters from the placenta, at which point the umbilical vessels separate from each other and cross between the amnion and chorion before connecting to the subchorionic vessels of the placenta (picture 1A-C). This typically occurs at the margin of the placenta (within 1 cm of the placental edge), but can also occur at the apex of the gestational sac. In monochorionic twins, the velamentous vessels often occur in the dividing membranes.