Yes, sperm can live in a womans body for up to five days, waiting for an egg. I don't think that a period will have anything to do with it, remember, sperm is microscopic. There is no sure way to know exactly how far along a woman is pregnant. A ultrasound can give a very good guess, but the doctors where I live always base it on when you had your last period. So if the first day of her last period was June 28 (example, I can't remeber what date you wrote) then I believe that they would say that she would be a month along on July 28. But just pay attention to how things go and how your wife behaves. If you have reason to believe that she is cheating on you, I would ask for a paternity test. Good luck, and hopefully things go well for you.
You need to relax and not get so worried then when the kid is born take a paternity test. Until then there is nothing more you really can do about it. God bless. Kimberly