Hi Fiona,
You can lose 1 twin and the other one will be fine. I have had two friends who this has happened to who have two heatlhy children now. I hope this isn't the case for you, as it would be very sad for you to lose a twin. I would get a repeat ultrasound and check that everything is going OK if you need to put your mind at ease. I will be having weekly u/s this pregnancy, so don't be afraid to ask for another scan, very easy for them to do. As mentioned above HCG (pregnancy hormone) can be an indicator. If you have lost a twin though, that may have been boosting your HCG and a small drop may not mean that the other baby you are carrying is not OK. I had very good HCG levels my last pregnancy for a long time, even though things were not growing that well, so a u/s is a much better and accurate way to go.

Goodluck with your continuing pregnancy, Meg