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Hi, It's Dr. C. Board certified Ob-gyne. I can help you today.
I'm sorry for the situation. With only 5 days between the sex, it's not possible to rule out guy A 100%. The chance that he's the father is lower mostly because you used pull-out. Some guys are very good at pull out. But there is research that supports the possibility of sperm being present in pre-cum. The other problem is that sperm can easily live for 3 days inside women with reports of 5 and even 7 days. The ultrasound has an error rate of 3 days.
So with the ultrasound giving the most likely conception date of Nov 2, we need to add a 3 days error to that. So that makes possible conception from Oct 30. And if for some reason sperm lived for 4 days, that could mean that even if one sperm survived from the Oct 26th sex, that this pregnancy could be from guy 1.
So overall, it seems much more likely it's guy 2. But because the sex days are so close, we can't say 100%. In order to be 100%, paternity testing would be needed. This could involve blood from you and blood or a cheek swab for either guy.
I hope this was helpful. Please reply if I can explain anything better. I want to be sure you get the most helpful information possible for your situation.