Because of the early ultrasound (where they couldn't really see much) and your 7 week ultrasound, as well as the fact that you have a period between the two, I would say it's guy B. Like Lovemykids says, sperm only lives 5-7 days max, so no way could it hang around for 3 full weeks.
The website below is really interesting, it documents what ultrasounds look like in the earliest weeks of pregnancy. Remember that on week 5, which is how far along you were during your very first ultrasound, the baby has only been alive 3 weeks since they count from your period, not from conception. I had an ultrasound at 5 weeks and all they saw was the yolk sac, they weren't even sure if it was a viable pregnancy. 1 week later at 6 weeks they saw an embryo and heard a heartbeat. By 8 weeks, which is how far along you'd have been if it were the other guy, you would have definitely seen something. In the earliest stages of pregnancy the baby follows a very specific development pattern, at most it'd be off a week, not three.
Hopefully this link shows up but if not message me and I'll try to get it to you another way.
http://www.baby2see.com/development/ultrasound_sonogram/first_trimester_scans.html#week4
Hey ! Im going through a very similar situation now. Did u ever figure out for sure who the father was... guy A or B ?
Hi ya
I was with man A up untill the 8th November we had sex that week. I started going with other fella man B say start of December my last period was like the 5th of December. I think I conceived after my period as im around 11 weeks tomarrow but not 100 % thow. Is there any way man A could be the father? I really hope that man B is the father. Thank you.
Im dealing with the sme situation rigt now. Its hard but my exact same situation. Dif dates mines guy a Nov 18... Guy b dec13.. Wit period rigt after guy a. & conception dates are closr to guy b.
Only way to know for 100% is to have a paternity test done when baby is born. But sounds like guy #2. Best of luck hun!
Sperm can only live in a womens body 5-7 days, guy A's sperm would of already died off
Yes Guy B is definitely the father since you would of ovulated around the time you had sex with him.