His kid

Me and baby daddy got back together first of January. I got pregnant that month. And found out in February. People are saying it’s not his I’m 29 weeks a few days. Right before we got back together I had sex with a guy right before Christmas or 2 days before. And if it was his I’d think I would be farther along but dr appointment says she’s measuring healthy and good. So would it be baby daddy’s or the other dudes baby?