The markedly macerated fetus at the left was delivered along with the placenta after the liveborn twin was delivered. The reason for the demise here is a "twin-twin transfusion" syndrome in which a vascular anasomosis between the two fetal circulations in a monochorionic twin placenta leads to diminished blood flow to one twin (the "donor") and increased blood flow to the other twin (the "recipient"). The donor may die for lack of blood, or the recipient may die from congestive heart failure.

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