A tricky fetal case of isolated transposition of great arteries without the I-shaped sign.

Isolated transposition of the great arteries (TGA) is a congenital heart disease that presents with severe cyanosis after birth and a fetal diagnosis is crucial for the preservation of life. The I-shaped sign (I-sign) is useful as a fetal screening method for TGA. We herein present a tricky fetal case of isolated TGA with a side-by-side position of the great arteries and no I-sign. Severe cyanosis immediately after birth necessitated urgent interventions. A potentially fatal outcome was prevented by a prenatal diagnosis. In the fetal diagnosis of isolated TGA, it is important to not only detect the I-sign, but also conventionally examine the ventricular outflow tract.