Okay! It says on Brooke's birth certificate in episode 2 of season 4 that her birthday is early March (I think March 5th or something... Around the time Haley and Karen find out their pregnant) so I developed my own hypothesis! They did get pregnant in October but didn't find out until 4 months in because :
Haley: maybe she was on the pill where you only get your period twice a year so it took a while for her to notice!
Karen: because her soul mate tragically passed - maybe she didn't notice that her flow was late or assumed it was just from grief and stress!
I'm satisfied with this! Who else is?
Any better suggestions!?