If mom is behaving badly, you may have to step out of your comfort zone with her and deal with it directly.
Be very clear that it is hurtful to you that she can't be supportive or find one nice thing to say about the pregnancy. I'd see what she has to say-- if she turns it on you (accusing you of inconveniencing her, making her worry, or acting like a martyr about it) or apologizes. That says a lot.
(I've been through similar experiences with my mom, however, she is very mentally ill so it could be a profoundly different kettle of fish.)
I think Dawn and AMom2 both had good advice if you are wanting to stay in relationship with your mom.Their methods would be my first route. Reverend Ruby also had an excellent suggestion of getting further resources if need be.
Even if it's not narcissism (and I'm not saying it is... my experience is only my own), she may have an extremely high level of anxiety. Is she able to express positive feelings and support at all? Does she only give it to other family members, but not to you? Just some things to think about...
And at some point, if you are hormonal and she's still dumping on you-- well, if you lose your patience and dump on her, so be it. **YOU are not a landfill for her negative emotions**. SHE does not get to put her feelings above or in front of anyone else's feelings. Your pregnancy is not, and should not, be about her. She should have either said "congratulations!" to you or she should have said "oh, something's boiling over on the stove, I'll call you back" and gotten off the phone and vented her 'worries' to someone else, like a good mom would do. She should be letting you enjoy your pregnancy...
And to that, congratulations. I hope you enjoy it thoroughly!