Hi there.
Firstly, Congratulations on you're twins

. I had my first IVF cyle, and am currently just over 9 weeks pregnant with twins myself.
I don't really think that I can help you with your question, but this is what happened with me, just incase it clears anything up for you....
In my case, when I went for my first scan, they found a gestational sac with the yolk sac which was ok. They also found a second one, also with the yolk sac, but it was very small and the nurse who was scanning me told me that as the babies were too small to see at that point, I should just consider myself pregnant with possibly one baby - but not to think too much about the other one, as it probably would not get any bigger... As you can imagine, after the trauma of having to go through IVF in the first place, I was not to impressed with the nurse and told her not to right of my baby until they could at least see that there was nothing there!!! ( I don't think that she liked me very much!!! )
The following week, I went for another scan and they said that there were two babies there.

. they said that one was 10mm, but the other one was very small, maybe about 5.4mm. They said that they had to keep an eye on the smaller one as it was very small.
I went for a third scan last thursday, and they said that one was 19mm and the little one had grown to 17mm, so had caught up with big baby
I know that isn't anything like your situation, but I talk to my babies everday and tell them that I am so blessed to have them. I believe that they have gotten stronger by the day because they know that mummy is a nutter who keeps talking to herself and wishing them to get bigger and stronger!!!
I hope that everything goes ok with you're little ones, just let them know that you want them to get big and strong and healthy, and that you can't wait to see them.
Take care.