R. - I was your friend. My husband and I tried for 6.5 years before we got pregnant. My sister got "knocked up" when she was 19 and it almost killed me. I can't tell you how bad I hurt over that.
But, the people I worked with knew what was going on. So one day, when my office mate/friend came in with a very serious "I need to talk to you" as she closed the door, I can say I was worried!
She sat down and told me that she was expecting, but before she told anyone else at work, she wanted to make sure she told me and that I could deal with it. She said that she and her husband were very excited, but she didn't want to hurt me.
I was so happy for her. And, I can't tell you how I appreciated her honesty. I appreciated her taking the time to make sure I knew "first". It was very special to me that she had thought about me.
Another co-worker wanted to have kids, too and when I finally found I was expecting, I did the same thing. I took her a flower and sat down and told her that I was pregnant and wanted her to know first.
You don't mention how long your friend has been trying. I realize some people think a year or 2 is a long time. But I also know the heart ache and pain of years and years going by and ....nothing.
So, my advice is to call her and tell her that you have some news to share, but you are worried about how she will take it and that you don't want her hurt. Please, please don't tell her "It'll happen for you" - I know people mean well when they say that, but for someone who has spent years trying - years crying themselves to sleep due to the emptiness created by this desire it comes across as totally snooty. I don't know any other nice way to say it. It frequently feels more like "neener neener boo boo" than anything else.
Talk to your friend from your heart - face to face would be best, but not always possible. Regardless of the how, just make it totally from your heart. She may still be hurt, but so much less than all those lame sayings people throw around that sound so fake.