But it need not rob one ounce of joy out of your life or your child's unless you let it. If breaking the news of your pregnancy is such a big deal to you that having the news leak out makes you feel "robbed" and worry about a "chance you will never get back", then maybe your mom's drama-seeking has influenced you, and it's a good opportunity to look at how it makes her act and reject it.
I have to completely disagree with this. Someone wanting to share there news with the rest of the world first isn't a drama seeker. Someone who wants to share everyone else's news with the world, especially if they have been requested not to, is a drama seeker and spot light hog. No one should expect the spot light, but no one should steal other people's spot light moments continuously either. The OP's mother is the one with the problem, and the OP has every right to feel the way she should and should take proper steps to build defenses so she doesn't feel it again.
I'm sorry I did not express myself better. I did not in the least mean that I think OP is a "spotlight hog," and if that is what came across I worded it very badly. What I meant was that there are very important aspects of what her mom did - the violation of privacy, trustworthiness as pp's pointed out, etc. I think the mom needs strong boundaries set with her, but the importance of the "reveal moment" is, in my opinion, not the most essential thing here.
I also wanted to encourage the OP not to get sucked into a competition with her mother for "ownership" of big family moments. It is a destructive head-game that nobody can win. When you are dealing with a person who is that self-centered and without any self-control, you will never, ever, get them to believe it is someone's else's turn to have the spotlight, and they are very likely to compete fiercely for it in increasingly destructive ways.
So if you just stop playing the spotlight game entirely, you get to set boundaries and not waste energy looking for "your turn" or trying to protect your turf, but just enjoying your life.