It's understandable why you're upset, they sound like a load of mercenary buggers, and targeting you after you've been through IVF and with everything you have going on right now is just horrible. You may feel like you can't do anything about it, but bear in mind that discrimination against pregnant women IS a big deal and you can do something about it if you feel you have a case.
I'm afraid I'm the unambitious one in our set up, but if your career is important to you, your partner should at least try to understand where you're coming from. But then again you've probably been through a lot going through the IVF and it inevitably takes its toll on your emotions and body, perhaps he's just trying to take some stress off you?
You're obviously good at your job and that's why their words have got to you so much, but try to keep in mind they may just have ulterior motives. Does your boss have any family or friends in the company he/she is trying to give a leg up?
Sorry, not very good with careers advice. But just remember, if it all goes wrong and you do decide to leave, wee in the watercooler before you go.