I had IVF done as well and I am going on 23 weeks with twins (boy and girl). Just be patience and listen to everything the doctor is telling you. I just bought my cribs last week so no need to rush on the cribs. They doctors told me the same thing about the heartbeats. I prayed and let God take control over the babies and then their were two :) Take Care and Keep us posted!
When is your next u/s? They saw two heartbeats at my 5 week u/s. Just like with a singleton, take it easy and take super good care of yourself!!!!!
We did IVF as well and had our first ultrasound at 6 weeks. They called them viable at that point with heartrates in the 160's and sent us back to our regular Dr. with good wishes. I think they want to see the heartbeat before they comment too much. You can't change the outcome, if anything happens it is just natures way. Just eat healthy, take your vitamins and take care of yourself. Hopefully they have you booked for an u/s in another couple weeks and you'll have great news! Good luck!
Probably not too much you can do this early. Just take things easy. Don't do heavy lifting...that sort of thing. I was six weeks when we had first u/s, and saw heartbeats, but it wasn't until our 8 week u/s that the doc was comfortable enough to say that both were looking strong and viable. Good luck!