At their very core, menstruation and, by extension, PMS are hormonal events, controlled by the fluctuation of hormones in a woman’s Four hormones work together to control a menstrual cycle:

• Follicle-stimulating hormone (secreted by the gland)

• Luteinizing hormone (secreted by the pituitary gland)

• Estrogen (secreted by the ovaries)

• Progesterone (secreted by the ovaries)

The levels of these hormones rise and fall during the month, depending on their levels, they can trigger PMS symptoms. Chapters 5 through 8 discuss the most common PMS symptoms.

The Cycle Starts

At the start of the period, on Day One, when bleeding FSH and LH are at their lowest levels of the entire cycle, which cause low levels of progesterone and estrogen. This first half cycle is known as the follicular, proliferative, or estrogenic because this is when estrogen, secreted by the ovaries, begins and triggers the growth of the uterine lining.

Mid-Cycle

Mid-cycle, as ovulation approaches, hormone levels continue to increase: LH and FSH levels rise, as does the level of estrogen. When the follicle starts to mature, the ovaries secrete a surge of progesterone in addition to the estrogen. This triggers ovulation, and this is when LH, FSH, and estrogen are at their highest levels. Depending on the length of a woman’s cycle, ovulation can occur on Day Seven (in a twenty-one-day cycle), Day Fourteen (in a twenty-eight-day cycle), Day Twenty-one (in a thirty-five-day cycle), or somewhere between.

Ovulation and the Luteal Phase

Once ovulation occurs, the luteal phase of the cycle begins. It lasts about fourteen days. Immediately, FSH returns to its base level while estrogen and LH begin to fall more gradually. In contrast, progesterone increases. Progesterone is secreted by the released follicle, now known as the corpus luteum, so that the lining in the uterus will produce fluids that nourish the egg. If a woman becomes pregnant, the levels of estrogen and progesterone stay elevated to maintain the pregnancy, but if no pregnancy occurs, progesterone falls and causes the endometrial lining to shed. Thus the period begins. In essence, the levels of estrogen and progesterone rise at different rates during a woman's menstrual cycle to optimize pregnancy and implantation.

The Chain of Events

The cascade of events between the organs and hormones involved in menstruation goes something like this:

• Hypothalamus releases FSH-RF to stimulate the pituitary gland.

• The pituitary gland releases FSH.

• The ovaries begin to grow follicles, which release two hormones, estrogen and progesterone, into the woman's bloodstream. When the follicle matures, the ovaries release a surge of progesterone.

• The estrogen and progesterone stimulate the hypothalamus to send out two chemical messengers: FSH-RF and luteinizing hormone—releasing factor (LH-RF).

• In response, the pituitary gland sends out FHS and LH simultaneously.

• The surge of LH causes the most mature follicle to burst and release an egg. This is called ovulation.

• The follicle, now called the corpus luteum, secretes in decreasing amounts and progesterone in increasing amounts.

• Under the influence of progesterone, the uterine secretes a fluid that nourishes the egg.

• If pregnancy occurs, the egg attaches itself to the wall. The corpus luteum continues to release progesterone and estrogen, and the body produces a hormone called (which is what early pregnancy tests detect in a woman’s urine).

• If the egg is not fertilized, the corpus luteum degenerates. The falling levels of progesterone cause the spiral arteries the endometrial lining to close off, stopping blood the surface of the lining. The blood pools in the lining eventually bursts it. This blood, along with the endometrial lining, forms the menstrual flow.

• FSH, LH, estrogen, and progesterone drop to base levels.

• When estrogen falls below a certain level, the hypothalamus once again stimulates the pituitary gland with FSH-RF.