Menstrual Cycles Vary
If you paid attention during your sex ed classes, you may recall that the menstrual cycle is supposed to last 28 days. In the middle of the cycle, the ovary releases an egg. If that egg is fertilized, it will ultimately implant itself in the uterus and become an embryo. If the egg is not fertilized, the uterus sheds the lining it has built up. That shedding is the "period," and it lasts about seven days. If the menstrual cycle can be determined through this schedule, then women should always know when they're getting their periods, right? So why does your girlfriend occasionally send you to the store at night to buy her tampons?
Those numbers are just averages. Some women have longer menstrual cycles than others. Some women have periods that last just a few days, while others have periods that last for a full week. To complicate matters further, their cycles and period lengths can change over time and can be influenced by a wide variety of things, such as medication, exercise habits and stress. Many women have a general idea of when they're supposed to get their periods, but these complexities are part of why birth control pills were invented. The pill suppresses ovulation entirely and controls the cycle with artificial hormones. It not only reduces her chances of getting pregnant to almost none (when taken correctly), it also provides regularity.