Vanishing twin syndrome begins like any pregnancy with multiples — except that one of the twins vanishes. The result is a pregnancy that goes on as a single.
The vanishing twin aborts spontaneously, typically during the first three months of pregnancy. The remaining fetus will absorb the rest of the material.
Most women find out that they are having twins before the end of the second month of pregnancy, but the heartbeat is likely to disappear before the third. This is when most women learn that the twin has vanished. This is an incredibly common condition, and it does not mean that the pregnant woman has done anything wrong.