The menstrual cycle is divided into phases. Each phase describes events that happen in the ovaries that lead to ovulation and menses.
The first half of the menstrual cycle is called the follicular phase. During this time, the ovary makes a follicle, a small unit that houses the egg and releases estrogen. Hormones from the pituitary gland coordinate events so that when it is ready, the egg is released (ovulation). The length of the follicular phase may vary. This is why there can be differences in a womans cycle from month to month.
After ovulation, the second or luteal phase of the cycle begins. The follicle becomes a corpus luteum. This is a small structure that continues to release estrogen. It makes progesterone too. If the egg is fertilized (the beginning of pregnancy), the corpus luteum lasts several weeks. If pregnancy does not happen, the corpus luteum dissolves, hormone levels go down, and menses starts. In normal cycles, the luteal phase is consistently 14 days long.
Menses is the most obvious event of the cycle, so it is used for counting. The first day of the period is day 1 of the menstrual cycle. Ovulation happens 14 days before menses begins. Still, the time of the next ovulation after a menses can vary by several days. So if you are tracking your cycles to determine when youre fertile, you must count your cycles for several months. You then determine the beginning and end of the fertile period by subtracting 18 days from your shortest cycle, and 11 days from your longest cycle.
If your cycles vary, there are more reliable ways to know when youre ovulating. One way is to monitor your basal body temperature (BBT). A very accurate thermometer is used to measure your temperature upon awakening. The BBT is plotted daily on a graph. With ovulation, a rise in temperature of about 0.5-1 degree happens. An even more accurate method is to use kits that measure changes in hormone levels in the urine that happen just before you ovulate.