The sooner the better. With something like a pregnancy, you need to tell her soon, because although she'll feel mixed emotions, I believe she'll want what's best, and your mother, seeing as she IS a mother, will have tips for you. I know this is very hard for you, but it's going to be hard for her too. Give her some space and time, and remember, if she gets mad straight away, that's not really her, it's her emotions, and in order to help you better, just try not to get too worked up. What's done is done, and you must now walk this new path. Be brave. Stay strong. :-)