I agree with Slanan but would say she made the mistake really. Though I'm sure we can understand.
She's managed to get to the stage of accepting herself. Then to the stage of you accepting her. But her self doubts remain very strong. That would seem to be perfectly normal.
Sure, seh wants to appear right. But eventually, she will get to the stage that every transisioning woman and man needs to, not worrying about what the world thinks. The only person who really matters, to her, is you.
Having said that, You should not feel bad that she is taking time to get that notion into her head. But your mother is just another positive affirmation.
Now may I strongly advise you you get on with your pregnancy? The problems your partner is having aren't going to go away regardless.