The umbilical cord inserts into the placenta in about 99% of all singleton (one baby) pregnancies. In about 1% of cases, the umbilical cord stops short of inserting into the placenta, and has the vessels travel between the amnion and chorion before the vessels finally insert into the placenta. The incidence of this anatomical problem increases to about 8% in the case of twins. This type of vessel insertion where the vessels insert into the placenta free of Wharton’s jelly is known as a velamentous insertion.
The velamentous insertion is diagnosed on ultrasound, and can be diagnosed as early as the first trimester. The pregnancy usually progresses normally, but the danger of fetal exsanguination exists at the time of labor due to rupture of the unprotected vessels. Your medical provider may discuss the option of cesarean section with you prior to the onset of labor to avoid the potential risk of rupture of the vessels during labor which would be life threatening to the fetus.
There is a condition known as vasa previa, which occurs as a result of the velamentous insertion, where the exposed umbilical cord vessels are present between the presenting part of the fetus and the cervix. In this situation, the vessels can rupture spontaneously or they can rupture at the time of amniotomy (artificial rupture of the membranes usually by using an amnihook). Vasa previa is a potentially fatal condition.
Other complications from the velamentous cord insertion include intrauterine growth retardation (IUGR) and also twin to twin transfusion in the case of twins. Twin to twin transfusion is where the blood is shunted from one twin to the other leading to one large and small twin which can be life threatening to one or both twins.
Wishing you a safe and memorable delivery. Pablo