The menstrual cycle is not always 28 days
A menstrual cycle normally lasts between 21 and 35 days, even though 28 days is often treated as the default. It is also often less regular near puberty and near menopause.Menstrual Cycle: Why 28 Days Is a Myth
Even normal-looking cycles may skip ovulation
Some menstrual cycles look normal from the outside but are not ovulatory. In other words, the body may not release an egg, or the time after ovulation may be too short for normal fertility.Menstrual Cycle: Why 28 Days Is a Myth
