Some pregnancies start off with two ‘sacs’ but then one of them either gets miscarried or reabsorbed, leaving only one viable foetus. This is known as ‘vanishing twin syndrome.’
It is relatively common and something that is being noticed more these days due to early scanning. If you are told at a very early scan that you are having twins it is very stressful to be told at a later scan that one of them has now vanished.
Can vanishing twin syndrome be prevented? No. There is nothing that you can do to prevent one twin being reabsorbed. It is normally caused by an abnormality.
Vanishing twin syndrome normally occurs in the first trimester, which is why it is a good idea to wait until after the first trimester to tell everyone you’re having twins.