Thank you for all your good luck messages! I'm pleased to report I had my scan today and both babies are fine.
In fact one is a bit of a porker and is bigger than the average singleton at this stage! The other is close behind and is the same size as a singleton at 20 weeks. 
I didn't have any tests so this was quite a nerve-racking scan, but they both seem to have all their bits - arms, legs, heart (pumping away). The sonographer said she could tell what sex they are but we said we didn't want to know just yet........this may change!!
As usual twin 2 was bashing around and wouldn't keep still. 'His' head is stuck up twin 1's bottom and 'she' keeps kicking him to get him to move. Bless. Fighting already.....
It was amazing how much they had grown since our last scan at 13 weeks. We could actually see their eyes and ears and fingers and toes. It was great. We have two brilliant scan pictures which we will add to our collection.
Luckily because we are having twins we get to see them again in four weeks time! No doubt they will have grown again.
The only 'negative' things (and they aren't major worries just yet!) are that they found protein in my urine and also my placenta is quite big and its blocking the 'exit'! This just means that if it doesn't move (which I'm told it probably will anyway) I won't be able to have a natural birth - the hospital seemed quite pleased about this, they want me to take the 'too posh to push' route to avoid any complications.
They've sent my urine sample off to be tested and told me to drink lots of fluid and hopefully this should be ok too.
Phew....glad its out of the way. I feel I can sit back and relax a little now and enjoy being pg and getting fatter!
love nikola.x
