Acardiac twinning or TRAP-sequence is a rare complication of monochorionic twin pregnancy. We present a case in which fetal demise of one fetus of a twin pregnancy had been wrongly diagnosed in the first trimester as a vanishing twin. This resulted in the unexpected occurrence of an acardiac twin in the third trimester, culminating in an acardiac twin with a birth weight higher than that of the normal twin. A review is presented on therapeutic opportunities including ligation of the umbilical cord under ultrasound or fetoscopy. This case report highlights the need for correct and early ultrasonographic diagnosis of chorionicity and detailed ultrasonographic follow-up of twin pregnancies.