A few years ago I read in a Dr. Ruth type column about a woman who was having twins and then the one baby just disappeared. I never heard of this before, so I went onto a medical site to find out more. Apart from being told to see a doctor immediately - I was not pregnant! - I was given the information I needed.
Apparently, it is quite common in multiple pregnancies for one fetus to be 'absorbed' by the other one. Here is an excellent article about the causes and effects of the vanishing twin syndrome: http://americanpregnancy.org/multiples/vanishing-twin-syndrome/
This made me think about the psychological adjustment of the surviving twin. We all know how close twins, especially identical twins, are to each other. Research have shown that twins feel each others' emotions, etc etc, and if one should die, the remaining twin suffers more than when a non-twin sibling dies. But what about it if one twin dies invitro, like in the vanishing twin syndrome? Would the survivor 'feel' the loss of the twin who died before birth, later in life?