My parents divorced when I was young, and I can remember returning from visits with my (beloved) dad and his girlfriend (who I absolutely adored) and getting quizzed by my mum and my nan and really pressurised to agree with negative things they were saying. If I ever agreed or said anything even slightly negative about dad/his girlfriend it was met by lots of crowing/cackling and praise.
It was absolutely fucking awful. I'm now a 47 yr old woman and I can remember this as if it were yesterday. It was so, so hard.
Even if the mum isn't quite as overt about it, there could be subtle pressure to slag you/your DP off. I can remember my mum/nan particularly wanted to hear me be negative about dad's girlfriend, and this just sounds so similar.
It might just be faint disapproval if he doesn't slag you off, compared with loving praise and encouragement if he repeats negative things you've said.
There's also the point mixed in there that he's 13 and might not have been listening properly/might have misinterpreted something (that pregnancy example sounds as if it could genuinely be crossed wires).
Either way, I don't think it's about you, even though I completely appreciate it's deeply hurtful. It sounds like a child caught in the middle who is being pressured to "pick a team" and faces disapproval if he doesn't have anything bad to say about you.
I'm obviously guessing, but that would be my suggestion. Let DP speak to him, and don't play into their plan by becoming the nasty and cold stepmum. Try and remain the same warm person that you've always been - don't give any ammunition to criticise you, but do be extra careful about anything that you say and how it could be misinterpreted.
Sorry I know that's shit, but this will pass.