This is awful. Let’s understand though that it takes two to tango. If she’s telling the truth she’s only telling you now because you’re pregnant and about to give birth - and this is all interfering with her plans to run off with your husband while knowing previously anyway he was married with kids.
You can’t stop someone from cheating but you can choose not to get involved with someone you know is in a relationship.
Her motives for telling you are not altruistic. She’s as much of a scumbag as he is if it’s true.
If she’s lying, what could be her motive. Did your husband turn her down. Did he get her in trouble at work. Is she completely mental and doing this because she’s completely mental.
We don’t know yet. Do you know her full name ? If so, check out her social media. Just don’t like any photos or anything daft. If she’s full of herself and stupid enough to get involved with a married man, she’s probably got something on her socials to give that away. Match up the dates she’s got dressy photos of herself and gushing about being out but no oned tagged - to dates your husband was out.
If she’s still communicating with you - ask her for proof - just say - can I see the chats you had , photos he’s sent etc.Ask her if it’s still ongoing or if she / he ended things. Be nice to her for now.
You also need to prepare for the fact they she may have told your husband that she’s told you. If she’s angry with him for ruining the life she had planned - well, she’s probably going to throw it in his face.
Olay it safe for now be calm until you know the full story, once you do …
If she’s telling the truth you have to decide if you want to try and save your marriage or if you want to leave.
If she can’t prove with certainty that her and your husband were indeed doing the deed you need decide with your husband what to do about things.
See how he acts. For now, take care of yourself and let things pan out.