Well my first ultrasound was done when I was twenty weeks and she was measuring on schedule. I was having this doubt earlier today because i had an appointment today and she measured my belly. i'm measuring at 30 and I kind of felt I like I am a little big to only be 7 months. My doctor told me that it's just probably a lot of fluid around her. I felt and somewhat already knew that it was my first childs father baby.
The words "It sounds" got cut off in what I wrote above. It sounds like, based on your last period, conception was in mid July, but do you have any other confirming evidence, is what I was asking.
I am not a medical professional, but would assume that since you had a period in between both men, that only the man who you had sex with AFTER your last period could be the father.