This is a personal opinion thing, because I know everyone is different. I lost my baby in September so I know what it's like to lose a child. A friend of mine's husband let it slip that she was pregnant (she was only around 5 weeks when he said this)... she went to the doctor for them to tell her her pregnancy was pretty much over at 6 weeks. She hasn't mentioned since if she lost the baby or not. I don't want to ask her if she has, but I still want to be supportive. How would YOU like someone to support you if you were dealing with a miscarriage or the possibility of losing your baby? Obviously I'm not going to go blow up her facebook with "how are you?" every day all day, but I still want to be here for her.
Honestly, I didn't want anyone to mention it because it would bring the hurt and pain rushing back to the surface. It took me awhile to be able to talk about it and when others would bring it up out of the blue it would just bring tears. I would wait for her to bring it up.
Send her a simple email, text ,card, whatever letting her know that you understand from your own experience and that if she needs to talk you're here for her. Then wait. If she wants to share with you then she will when she's ready.
Personally, with my recent loss, I appreciated when people minded their own business. I appreciated it when it wasn't the first thing people were talking to me about. I appreciated when they were letting me bring it up and didn't ask questions about it. Mainly because it hurts...and also because I didn't wanted to talk about it to peopleI wasn't so close with or didn't trust...but there was also rumors that I entirely faked my pregnancy to get my ex back( which were obviously not true) and I knew that if I talked to the wrong people about it, what I would have said would have been deformed and more rumors on me would start.