I guess I should throw in there that he did not pull out, any of the times they had sex.....
Gosh, I'm sorry to hear about your boyfriend cheating on you! That stinks and I'm sure leaves you wondering if this guy is worthy of you.
If she ovulated late, then yes. He could have impregnated her but she wouldn't know yet. It would be very early. Timing wise, I doubt he impregnated her. If she turns up pregnant---- wouldn't she want the baby to be her boyfriend's? Your boyfriend would just sit quietly tight and ask for a paternity test after she delivers if she shows up pregnant. He doesn't have to do a single thing until then. This was a brief sexual affair. He doesn't really owe her anything unless there is a paternity test proving a child is his. He WAS extremely foolish and irresponsible, that is for sure. And I'd get a full STD screening if I were you.
But I guess you have to decide if you still want to be with him knowing he cheated. I'm really sorry hon. That hurts!
Kelsa, I don't know if you're pregnant or if you have children with this boyfriend, but if both answers are "no" - I think your "worst fear" should be that you are pregnant right now.
Don't hang on too tight to a failure - I know your heart is breaking, and you're really just hoping you can carry on this relationship with him as before, but once a cheater always a cheater. Especially when he did this multiple times and didn't even bother to use protection.