The menstrual cycle is divided into three phases: the follicular phase, the ovulatory phase and the luteal phase. The start of the menstrual cycle is the first full day of bleeding and is commonly referred to as cycle day one.

The follicular phase is the most variable in length depending on the individual, but usually lasts 13 days in a typical 28-day cycle. During the follicular phase, the pituitary gland located at the base of the brain releases two hormones: follicle stimulating hormone (FSH) and luteinizing hormone (LH) to stimulate the ovaries and cause growth of the follicles. The developing follicle secretes estrogen that allows the endometrial lining to grow and prepare for the implantation of the embryo. Estrogen also triggers changes in the cervical mucous.

The ovulatory phase begins with a surge in LH levels, which causes the follicle to rupture and the egg to be released from the ovary. In a typical 28-day cycle, this usually occurs on day 14, but will occur earlier or later depending on the length of a woman's menstrual cycle. Fertilization of the egg by the sperm occurs in the fallopian tube 12 to 24 hours after ovulation. The resulting embryo moves through the fallopian tube to the uterus, where it implants and continues to develop. NOTE: Taking anti-inflammatory medications (over-the-counter medications such as ibuprophen) can interfere with this stage and should be avoided at the time of ovulation.

The luteal phase occurs after ovulation. The ruptured follicle develops into a corpora luteum, an ovarian structure that produces estrogen and progesterone. These two hormones are necessary to develop and maintain the endometrium (the lining of the uterus) so the embryo can implant and grow. If fertilization does not occur, the corpus luteum stops producing estrogen and progesterone. Once these hormones are withdrawn, the uterine lining breaks down and sheds, which is the start of the next menstrual cycle.

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