In Vanishing Twin Syndrome (VTS) there is a twin pregnancy. However, at an early stage, one of the fetuses stops developing. Sometimes you will not know anything about it but, because scans these days take place at an earlier date, more and more women are becoming aware of it.
There may be a bleeding that may be associated with cramps. You may notice that you feel less pregnant. Sometimes there are no physical symptoms, and only a scan shows that there was a twin.
As the living child grows, they may absorb their twin, or it may be absorbed back by the mother.
Vanishing twins stop growing at an early stage, and therefore there are no known dangers to the living child or mother.
From Oliver In Pregnancy