Jenna,
I am so, so sorry for them.
From experience: call on the phone. Don't be afraid to talk about it with them. Don't worry you'll remind them. Believe me, nothing, absolutely nothing else will be on their minds for a very long time. Letters are great, but even a card is appreciated. I kept a scrapbook full of letters, cards, ultrasound pictures, everything we had pertaining to our son.
Refer to their baby by name, always.
If you don't know what to say, say that you don't know what to say. It's honest, and it's always better than saying nothing. Let them talk as much as they need to and don't change to a "safer" subject, even if it's uncomfortable for you.
Some of the hardest things to hear at a time like this may be "It's God's will" and "God never gives us more than we can handle" and "God needed this baby more than you did." I was a serious believer at the time of my loss, and honestly, I'd have rather been attacked with a butcher knife than hear that it was God's brilliant idea. The best thing to do is follow
their lead with any God-talk. If they're comforting themselves that it was God's will, okay, but other people declaring this can be excruciating.
Someone suggested a small remembrance box, and that's a lovely gift idea. So is a piece of jewelry with the baby's birthstone, or a sweet little ornament, perhaps with the baby's name. If you lived closer, I'd suggest a perennial plant or tree to honor the baby.
Don't bring up "trying again." Nothing will replace this baby, ever. One thing people trying to comfort the grieving sometimes forget is that they didn't just want a baby, they wanted
this baby. Although no one else knew this baby, they did. Your friend carried this child under her heart for nine months, and a big part of the pain of this loss will be that others didn't know her baby the way she did. I can't overemphasize the importance of respecting that, and Jenna, I know you'll respect it because you're one compassionate lady.
If/when they do try again, it's probably going to be much scarier (and take a ton more courage) than your average pregnancy, and stir up a storm of painful emotions. By no means will it mean they're over it.
Keep this child's birth date on your calendar and always remember that your friends will be grieving at that time of year, every year, even years from now, when they're happy, if/when they've had more children. They will
always grieve this child. Your acknowledgment of this in the years to come, your continued willingness to talk about it, will mean the world to them.