How to tell my (possibly infertile) friend that I'm pregnant?

My good friend has been trying to get pregnant for over a year and a half without success. I got pregnant within two months of trying. I'm really nervous about telling her, as over the last year some of her other friends/in-laws have gotten pregnant and she expressed sadness/annoyance about it. She recently told me that she thinks she's completely unable to have kids.

How do I break it to her gently? Or should I just act normal, like "surprise! I'm pregnant!" and be all smiles, like there's no issue at all? I've never been in this situation before, any advice welcome.