Hi OP, I'm an employment lawyer.
First and foremost please don't do what scaryclown suggested, it would be obstructive, would make you look like a nutter and wouldn't help your case in the slightest.
To answer your question, to discipline or dismiss in a he said/she said situation, all the employer needs to do is demonstrate that they genuinely believed in your guilt, on the balance of probabilities (i.e. is it more likely than not that you committed the "offence"). It's essentially a 51/49 type situation and there's no requirement to prove a case either way. But with three statements saying you did insult your colleague versus your statement saying you didn't, on the face of it they could fairly discipline or dismiss (assuming there aren't any other outside circumstances you haven't mentioned here).
Depending on what you said, unfortunately, it could be gross misconduct, but usually they would have to support that view with previous action in similar circumstances and/or something written into their disciplinary or bullying and harassment policies.
You're entitled to defend yourself at a disciplinary hearing (you could theoretically ask to cross examine the witnesses if there are holes in their statements). You need to prepare yourself as well as you can and take a work colleague as a companion and witness (assuming you're not in a union).
Being pregnant won't protect you if they genuinely believe that you are guilty and there is sufficient evidence for a reasonable employer to make that finding - the only way it protects you is if they fabricated the entire process because of your pregnancy.
Sorry, probably not what you wanted to hear, but better to be prepared.