Velamentous cord insertion
A velamentous cord insertion refers to the insertion of the umbilical vessels into the membranes portion of the placenta. A velamentous cord insertion occurs in 1.1% of singleton pregnancies; 3% in diamniotic/dichorionic twins; and 13% in monochorionic twins. Primiparas and women of advanced maternal age have a higher frequency of velamentous umbilical cord insertion. Placental ”migration” from the region of the internal cervical os occurs at a variable rate during the latter part of the first and throughout the second and early third trimesters. A first trimester cord insertion in the lower uterine segment should alert the sonologist that a subsequent third trimester scan may reveal a marginal placental cord insertion, velamentous insertion, or a vasa previa.
Pretorius and co-workers have defined a marginal placental insertion as occurring within 2 cm of the placental edge. They were able to visualize the placental cord insertion in 54% of second and third trimester ultrasound examinations. Several authors have found an association between velamentous umbilical cord insertion and pregnancy complications. There is a higher incidence of low birthweight, small-for-gestational age fetuses, preterm delivery, and abnormal intrapartum fetal heart rate patterns when a velamentous cord is present. Velamentous cord insertions have also been associated with a single umbilical artery. While there have been a few reports linking velamentous cord insertions to congenital malformations, the majority of anomalies are deformational in nature (i.e., hip dislocation etc), resulting from intrauterine constraint.
It is occasionally difficult to distinguish between an eccentric and a velamentous cord insertion. Manipulation of the uterus may stretch the umbilical cord so that the actual insertion site can be accurately visualized sonographically.
It is easiest to evaluate the placental cord insertion in the latter part of the first and in the second trimester.
In order to confirm that the cord insertion has been identified, the following techniques may be employed:
1. When the examiner taps on the maternal abdomen over the apparent cord insertion site, it will appear ”fixed”.
2. Color Doppler is helpful in locating the cord insertion and identifying the vessels that extend from the cord insertion into the placenta.