The menstrual cycle can be divided into two phases, the follicular phase and luteal phase. The follicular phase is comprised of the two weeks leading up to ovulation; the luteal phase follows ovulation and continues until the onset of menstruation.

Hormone-producing organs are called endocrine organs. For reproduction, women use primarily three of these organs: the ovaries, the pituitary gland, and the hypothalamus. The pituitary gland and the ovaries produce four hormones that constantly change in concentrations in order to facilitate fertility: follicle stimulating hormones (FSH), estrogen (E), luteinizing hormone (LH), and progesterone (P). Meanwhile, the hypothalamus, releasing GnRh, acts as a conductor, telling signaling to the pituitary and thus the ovaries when to begin and end production of these hormones. As hormones travel in tiny, potent quantities through the blood, they interact with receptor sites that coordinate functions of the endocrine organs. This constant dance of hormones orchestrates our fertility-a dance that changes throughout the month and throughout our lives.

The Follicular Phase (1st half of the menstrual cycle)

Your cycle begins with the onset of menstruation, also called the follicular phase. The hypothalamus, in response to the dramatic drop in estrogen and progesterone levels from the previous cycle, sends a message (via the hormone GnRh) to the pituitary gland for it to release a new batch of follicle stimulating hormones (FSH) to the ovaries. FSH stimulates the ovaries to produce estrogen and mature of a small group of follicles, one that will eventually spur an egg. (follicles mature into eggs) After about 12 days, the production of estrogen reaches a critical peak in the body - a signal that ovulation is ready to occur. However, this does not happen on its own. The hypothalamus is once again alerted and releases more GnRh to the pituitary gland. The pituitary gland now releases a different hormone, the luteinizing hormone (LH) to the ovaries. The luteinizing hormone is the trigger that will spur an explosion that releases the egg from the dominant follicle that has been maturing for the past 10 days. Ovulation occurs.

The Luteal Phase (2nd half of the menstrual cycle)

Directly following ovulation, the ruptured follicle from which the egg was released reconnects and becomes the corpus luteum. In preparation for fertilization, it produces estrogen and even greater amounts of progesterone to create an extra lining in the uterus. After about 12 days, if no fertilization occurs, the corpus luteum dies and estrogen and progesterone levels plunge. This causes the lining to be expelled and menstruation to begin.

(continue to Potential causes of PMS)