Ok what I'm about to say is hopefully over-reacting as I'm feeling triggered by your story. But not to tell you I feel would be irresponsible, if I'm right, as not telling you early enough could be detrimental to you. I've been reading posts on Mumsnet for a while but have joined in order to reply to you.
It's sounding suspicious. Join a union. Then make it known to them that you've joined one. If you can't join one, casually drop into conversation you have a friend with a legal or union rep connection (hopefully you have one, but if not think of me instead ;-) )
Keep a diary of what you told them when, and what they're telling you when. Be REALLY factual, but include how you felt afterwards. Print out/forward to home all emails that are in any way related to your employment status, pregnancy and performance. Look up and print off/email to yourself at home the disciplinary procedure, the pregnancy policy, and disability policy. These might be kept in an "employee handbook".
If there are any meetings, especially last minute ones or "can I just grab you a minute in private?" Type stuff, TAKE SOMEONE WITH YOU, even if it's someone from their HR dept (request if HR can be involved). Keep your own notes in the meeting, even if they're making their own, even if you feel they trust you. Send a follow up email to them summarising the meeting. If they refuse to let you bring someone with you, ask if you can record the meeting AS A REASONABLE ACCOMODATION for your pregnancy-brain. Link below contains useful information in any case...
www.dma-law.co.uk/is-it-illegal-to-record-conversations/amp/
Remember that HR are there for the company's benefit, not yours. But they will inform your boss if they're breaking any rules. But meet with HR anyway.
These mistakes you're making - send an email specifying which ones you made and what you're going to do to avoid doing them in the future.
I have an invisible disability and this is sounding familiar.... I have been sacked in the last year, from a medical establishment, during my probationary period. I believe in an underhanded way after I had a report back from Access To Work that suggested accomodations that were going to cost them £500, plus extra time for certain tasks that were written in might have to be permanent adjustments. I happened to make a mistake at the time that with anyone else they would have supported and trained, but they pinned it on that. In the "can you just pop upstairs for a minute, boss and boss' boss just wanna see you?" was my notice meeting. They said verbally that the amount of support they were giving me for the disability was unsustainable long term, then told me about the mistake I'd done and why (it is a fireable offense to be fair, but often taken with understanding). I wasn't given any disciplinary meetings beforehand or any warnings. In the letter afterwards it only stated the issues with the mistake. In probationary meetings, the minutes afterwards never specifically mentioned my disability, or any emotional struggles I was having, even though I talked about it and how I was working towards meeting my needs. My Union couldn't take on the discrimination case because there was a less than 50% chance of winning, because there was no direct written evidence of discrimination, and I did make that mistake after all and they made it all about that in official documents. In my new job (and on new medication) I'm doing much better.