The cocoon sign: a potential sonographic pitfall in the diagnosis of twin - twin transfusion syndrome.

OBJECTIVES: To investigate the frequency and clinical importance of the cocoon sign in twin-twin transfusion syndrome (TTTS).

METHODS: Charts and ultrasound images of all pregnancies with TTTS that underwent selective laser photocoagulation of communicating vessels at our institution over a 1-year period were retrospectively reviewed for a cocoon sign. TTTS had been diagnosed and prospectively staged if monochorionicity was suspected and the maximum vertical pocket (MVP) of amniotic fluid in one gestational sac was >/=8 cm, while the MVP in the other gestational sac was </=2 cm. The cocoon sign was diagnosed when there was an ultrasound finding of a donor twin with severe oligohydramnios enveloped by dividing membranes and connected to the uterine wall by a laminar stalk of these membranes.

RESULTS: The cocoon sign was noted in 9/58 (15.5%) cases. There were no differences in gestational age and stage of TTTS upon presentation, gestational age at delivery or number of neonatal survivors between the groups of TTTS cases with and those without a cocoon sign. One patient in the cocoon group had been misdiagnosed because the donor twin's MVP was measured from the donor's tethered membranes to the posterior wall of the uterus within the recipient's sac.

CONCLUSIONS: The cocoon sign is present in at least 15% of patients with TTTS. Its recognition is important to avoid misdiagnosis of TTTS. The practical implications and potential complications of this sonographic sign are discussed.