triiniodymus

A grossly malformed fetus with three heads, joined at the occiput, and a single body. [tri- + G. inion, nape of the neck, + didymos, twin]

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tri·in·i·od·y·mus (tri″in-e-odґĭ-məs) [tri- + inio- + -didymus] a fetus with a single body and three heads united posteriorly.