Gordon gave a great response. I just wanted to add my prayers that things will continue to go well. It sounds like you've got a great team of doctors and that is so reassuring.
This got long! My apologies! Ugh, and look at me talking like I know it all...I don't.
About trying to help keep your wife calm, I have two things to say...okay, maybe three?
1. If she has a "melt down", it is normal and not the end of the world. It ain't fun but the stress isn't going to make anything worse -just not any better.
2. It isn't your job to keep her calm - just try not to stress her out more. And anything you can do to releive some of the tension she feels will only make her feel better (and like you more!).
3. Being attentive, yet calm yourself, will automatically calm her too.
I'm sure you already know all that...
Some more tangeable suggestions to help her feel better:
Anything that can bring "normal" pregnancy thoughts and happiness such as a baby shower, buying baby things (online, via catalog, however she is able to do so), making/ordering baby announcements and getting them ready so all that needs to be added is the date/weight/name-type info, etc. Though some having had a previous loss might wait, I honestly can't imagine that it could hurt any worse, really, so in my thinking, she might as well try and enjoy it now. Though, I can understand the fear of opening yourself up after a loss...it all just depends on you and your wife.
And, as always, anything that shows either effort or thought -money aint everything! Having her parents write out their parenting bloopers, thoughts and experiences from her youth/babyhood, writing her a letter by hand telling her how you feel -both the good and the bad, but ending with love and hope, making a photo album out of the ultrasound pictures (though I would get high-quality copies and not use the originals), or any other kind of pregnancy memorabilia that she can hold on to.
But in all reality, what she really needs is someone to be there, someone to listen, and someone who cares...which it sure sounds like you are.
And last but not least, don't forget yourself. You know that whole airline shpeal about putting your oxygen mask on before helping someone else get theirs on? Don't forget that while you wife is having a hard time breathing right now, you can't help her if you aren't breathing yourself. Take time to do what you need to do so that you can truly be there for her.
Sending my prayers your way,