Prenatal vitamins and really long cycles!!

My husband and I started ttc in August. I haven't been on birth control for years and have a pretty regular 31-33 day cycle. I started taking prenatal vitamins about two months before we started ttc. Since then my cycles have switched to 41-45 day cycles. This month I decided to stop taking them and I went back to a 31 day cycle. Has anyone else had this issue? I have tried different brands (rainbow and nature made). I would obviously rather have a 31 day cycle so that I have more chances to conceive but know the importance of taking PNV's. Any ideas or thoughts on this one would appreciated!