We split at Christmas and she is back with her ex , but pregnant

[Foolish man]

Hi
Super quick summary. Ex girlfriend left me for a married man , split his family , but they fought a lot and he was violent on some occasions. My ex and I still kept in contact , on and off throughout those 8 months . She left him this summer and then we got back together. We found out she was pregnant then everything changed about her from that point on . All intimacy stopped, she was rude and very argumentative . My children didn’t like her and one refused to stay whilst she was here . After a few rows before Xmas she left . Saying she was moving back to her mum and dads .

However I now know she is back with the married man ( who is getting divorced) .
If the baby is mine , I will do a dna test after birth , I would want to be a father and involved .
But communication is difficult as she is lying to me when she is with him , he hates me and will always  cause problems .
First scan coming up and I want to go , check dates etc and see the scan , but she is avoiding telling me when it is , but not saying I don’t want you to go .
I need to understand best how to communicate without it descending into arguments , but I feel I can’t keep letting her lies go without challenge

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