OK so the love of my life is 18 weeks pregnant and not to long ago she had told me she had unprotected with another guy... but from what the doctors told us is that the baby was conceived on arpril 19 2010... but she had that unprotected with that other person 3 and a half weeks before that... but within that those 3 weeks we've been having unprotected sex... and every time I had an orgasam in her... I just want to know what are the chances of this baby being mine??

There is no way to know if the child is yours at this point. A DNA test after the baby is born is the only way to know for sure.