Something else to be aware of is that *any* sort of stress on your system, whether emotional or physical, can delay ovulation. Then unless you know when you ovulated, you will notice a late menstrual period; if you're sexually active, you might suspect pregnancy.
I've always been pretty regular, but one year I was moving at Christmas and very busy at work, and I did not ovulate at all that month or get my period. My sister went without her period for several months after she went off birth control pills. She had come down with a bad case of strep throat. (Which is weird, because although she is a teacher, it was summer and she was working from home and seeing practically no one!) She thinks that the stress of the infection and treatment may have delayed the return of her period even further.
I suggest that *all* women (unless they're on hormonal birth control) chart their menstrual cycles! By taking your BBT (basal body temperature) every morning until you are sure you're ovulated, you'll know much more precisely when your period is due. You can get more info by looking up the Fertility Awareness Method (FAM).