The sonographic analysis of amniotic fluid in twin pregnancies can be complex and difficult, particularly when one twin has oligohydramnios or anhydramnios. This article describes a pitfall, the "amniotic wrinkle," which can lead to the erroneous impression that both twins have adequate fluid when one actually has little or none.Sonograms of twin pregnancies in which the sonographer's initial assessment was of adequate fluid for each twin but imaging by the author on the same day showed one twin to have oligohydramnios were analyzed to identify recurring image patterns related to the intertwin membrane that may create this misleading impression.With oligohydramnios of one twin, the intertwin membrane may become redundant, folding on itself, creating an amniotic wrinkle: a short linear structure extending perpendicularly away from the twin with decreased amniotic fluid in toward the amniotic space of the other twin. A variation of this occurs when this fetus moves an extremity into the fold, with the two layers of the fold apposing each other between the limb and torso or between two limbs. An additional pattern is also described: an intrauterine sling or "cocoon" in which a fetus appears to be suspended within the amniotic space of the other twin.To avoid pitfalls in assessing amniotic fluid for twins, the intertwin membrane should be shown in every image that is used to document fluid ascribed to each twin, and images at right angles to the initial image can help identify an amniotic wrinkle.