Wickedlady's advice is good I think.
I would write a letter to her, explaining that you are keen to be involved, but acknowledging that it needs to be on terms which suit you both.
You could see if you can negotiate a way for you to be involved which respects her wishes too. Maybe she could update you by text after midwife appointments/scans etc. You could also say that perhaps closer to the time of the birth you could get together to think about what would work in terms of you being in the baby's life.
I agree with what others have said about midwife appointments actually being quite intimate, and maybe not ideal for an ex to attend - you wouldn't ask her to come to the GP with you!
You obviously feel strongly about this, particularly given your previous experience of losing a baby. But it really is important you keep cool, act rationally and calmly, and don't rush into things. She will be pregnant for 9 months, which is a long time to sort things and for things to settle. Don't act too fast - sit back, talk to other people, and think carefully before doing anything.
This isn't about you, her and her new partner - this is about a new little human, and you all need to act in his/her best interests. Your ex may be unreasonable (or not, we've only heard your side!) but you can increase the chances of her being reasonable by being reasonable, cool, calm and collected yourself.