I think, like an earlier poster said who had a very prem baby, I think it is very, very important that the life of the baby is acknowledged, this is a gesture of hope that would be very much appreciated.
I lost a baby very late in pregnancy and although it isn't the same thing, while my baby was struggling (and after it died) I really appreciated the cards I got, which were kind and wise. One thing I noticed that was lots of old people, some who I didn't know very well, knew exactly what to say and do, having suffered losses, struggles in their lives, and some younger people got it very wrong. I still have some of these cards and some presents I was giving (some were buried with her) in a special box and they are very important to me
People who were too intrusive i.e. phoning me up or, much worse, wanting to visit I found very upsetting (and in some cases made me angry - why couldn't they just have written?). I was having to hold onto myself and deal with what was happening in front of me and some people (not closest friends and family) wanting actual contact, to ask how things were etc. was just too too much.
If I was in her position, right now, I think I would like a very kind card, that offers support and some kind, kind (and positive words) and perhaps a tiny teddy or something like that for the baby (which is the hope thing).
I think visiting would be too much, although of course you can write in your card that you would love to visit when the time was right.