The menstrual cycle is a recurring cycle of physiologic changes that occurs in reproductive-age females. Overt menstruation (where there is blood flow from the vagina) occurs primarily in humans and close evolutionary relatives such as chimpanzees. The menstrual cycle is under the control of the endocrine system (system of glands that involve the release of hormones). It may be divided into several phases: menstruation, the follicular phase, and the luteal phase. Ovulation defines the transition from the follicular phase to the luteal phase. Counted from the first day of menstrual flow, the length of each phase varies from woman to woman and cycle to cycle. The average cycles length is 28 days.
1. Menstruation is the most visible phase of the menstrual cycle Menstruation is the shedding of the uterine lining (endometrium). It occurs on a regular basis in reproductive-age females of certain mammal species.
2. The follicular phase: increasing amounts of estrogen , menses begins to cease and the lining of the uterus thickens.
Follicles in the ovary begin developing under the influence of a complex interplay of hormones, and after several days one or occasionally two become dominant (non-dominant follicles atrophy and die). Approximately mid-cycle, 24-36 hours after the Luteinizing Hormone (LH) surges, The dominant follicle releases an ovum, or egg in an event called ovulation. After ovulation, the egg only lives for 24 hours or less without fertilization while the remains of the dominant follicle in the ovary become a corpus luteum(latin for “yellow body” is a temporary endocrine structure in mammals, involved in production of progestogen, which is needed to maintain pregnancy).
3. The luteal phase: also called the secretory phase or post-ovulatory phase.
An important role is played by the corpus luteum, the solid body formed in an ovary after the egg has been released from the ovary into the fallopian tube. This body continues to grow for some time after ovulation and produces significant amounts of hormones, particularly progesterone. Under the influence of progesterone, the endometrium (uterine lining) changes to prepare for potential implantation of an embryo to establish a pregnancy. If implantation does not occur within approximately two weeks, the corpus luteum will involute, causing sharp drops in levels of both progesterone and estrogen. These hormone drops cause the uterus to shed its lining in a process termed menstruation.
It is generally accepted that the time from ovulation to menstruation is always 12 to 16 days, whether your cycle is short, average or long. But while this phase does tend to be more constant than the phase before ovulation, recent research suggests it may range from 7 to 19 days.