It's very difficult, but I think it's best to tell her sooner rather than later. Because we were having fertility treatment and had been trying for 3 years, people tip-toed round me a lot and when I got married in October one of my husband's friend's girlfriends dropped out and didn't come at the last minute, saying she was ill. I found out later (much much later, like two weeks ago) it was because she was 5 months pregnant and didn't want to upset me. I found it a lot more upsetting that everyone was keeping their happy news from me, it made me feel all lonely and like an outsider. She might want to have a little cry when she first finds out, but then I'm sure she'll be glad you told her and will be dead chuffed for you. It just hurts a bit at first because it reminds you so much of what you want, but can't have.