Sonographic pitfalls: the cocoon sign

TTTS. Sonographic pitfalls: the cocoon sign

Because of the presence of oligohydramnios, the donor twin is usually stuck against the walls of the uterus: thus, the name ‘stuck twin.’ However, in approximately 15% of TTTS patients, the donor twin is enveloped by the dividing membrane, such that it is connected to the uterine wall by a stalk of these membranes (Figure 7.5a–c). 

As a result, the donor twin, despite having anhydramnios, may not be stuck to the uterine wall. We have called this potential sonographic pitfall ‘the cocoon sign’, as the term implies.20 Thus, a non-stuck donor twin may not necessarily be better off than a stuck one. 

The importance of the recognition of the cocoon sign lies in the ability to correctly diagnose anhydramnios of the donor twin and not measure fluid of the recipient twin mistakenly as belonging to the donor twin, by placing the calipers from the edge of the enveloping membrane to the uterine wall. Because the donor is allowed to move, albeit enveloped by the membranes, the sonologist may erroneously conclude that the fetus does have ample amniotic fluid.