I had to do this and know how you feel. What i did was break it to her a little at a time. I first told her i missed my period...but i an waiting to take a test until next week. then a week later i told her i was infact pregnant. We are very close and i am considerate of her struggle to get pregnant so i don't always talk about the pregnancy.....that's just what i did.
Hi. I was that mother who lost her child and don't hide it from her is my advise. I had a situation similar to yours and yes it hurts, but everything hurts at atime like that. My friend gave birth to her beautiful baby boy 6wks after I lost my beautiful baby boy, he was the only baby I could bear to be around because she had struggled with a difficult pregnancy and this boy born with Down's syndrome was a sign of hope to me even at the worse time in my life
PS.... A year later I gave birth to my gorgeous daughter who amazingly also has Down's.. She has healed a lot of my pain, although it's never far away, your friend will always carry that pain, but she will be pleased for you.
Don't hide it but prepared to have this relationship cool down. Your friend may need some space and time.
I agree with Ecologic that this will be hard for her and she may stay away for awhile. It doesn't mean she is not happy for you, but it is certainly a reminder of her tragic loss. I too would not hide it from her, but be gentle with the news. Try not to discuss it in her presence. Congrats on your pregnancy, and I wish you all the best!