Menstrual cycle length varies from woman to woman and from cycle to cycle but is typically between 23 and 35 days. Ovulation usually happens 12-16 days before the next period. Many women think that they ovulate on day 14 but this is just an average. Most women will actually ovulate on a different day of the menstrual cycle, and this will also vary from cycle to cycle. In fact, 46% of menstrual cycles vary by seven or more days.