The Stand Fans..time line issue?

I love this movie really, but one thing that has bugged me is the time line for when Fran gets Pregnant. She finds out on July 17th right after meeting Stuart. She says she is due in January which puts her pregnant before the flu. But when she has the baby the Doc says "when two parents are immune we will see the last of Captain Tripps." Now the baby isn't Harold's because he claims to be a virgin when he meets Nadine. So who's baby is it? LOL. I know I am over thinking and its just a movie but its really bugged me for years now. LOL