The menstrual cycle causes many changes in hormone levels and the condition of the endometrium (the internal lining of the uterus that allows for the implantation of a fertilized egg). It is called a "cycle" because it repeats each month in preparation for pregnancy.

It is interrupted by pregnancy, which is part or normal reproductive physiology, but it can also be altered or even absent with hormone abnormalities, medications, and menopause. Also, it can be altered via more subtle mechanisms involving the emotional effects on the adrenal glands, the thyroid, and the limbic system of the brain; even stress can interfere with the cycle.

The menstrual cycle is divided into three phases known as

1. the menstrual phase,
2. the follicular phase, and
3. the luteal or secretory phase.

There is considerable overlap between the menstrual phase and the follicular phase, since follicle stimulation begins before the menstrual bleeding is actually finished.

• The menstrual phase lasts roughly from day one to day five,
• the follicular (estrogenic) phase lasts roughly from day six to day fourteen (not counting any overlap within the menstrual phase), and
• the luteal (progestational) phase lasts roughly from day fifteen (after ovulation) to day twenty-seven.

OVULATION separates the follicular phase from the luteal phase at "mid-cycle."

Because estrogen is the primary hormone during the follicular phase, it is also called the estrogenic phase; because progesterone is the primary hormone during the luteal phase, it is also called the progestational phase. As such, during the follicular phase, estrogen is the primary hormone; during the luteal phase, progesterone is the dominant hormone, although estrogen continues to be present and rises during this phase as well.

 

Negative Feedback Loop

The menstrual cycle depends on a negative feedback loop that involves the

• ovary,
• pituitary gland (which stimulates the ovary), and the
• hypothalamus (which stimulates the pituitary gland).

This is known as the hypothalamic-pituitary-ovarian axis. A negative feedback loop is frequently used in the body's endocrine (glandular) system to maintain homeostasis (balanced physiology) or cyclic functions (as in the menstrual cycle, thyroid function, insulin-glucose interaction, and other endocrine fluctuations).

The negative feedback loop (hypothalamic-pituitary-ovarian) axis in the menstrual cycle:

PRE-OVULATORY

When estrogen is low, the hypothalamus stimulates the pituitary to release hormones which stimulate the ovaries to produce estrogen via FSH (follicle-stimulating hormone). This begins the process of maturing a number of fluid-filled sacs in the ovary known as follicles. One follicle outpaces the others and predominates, producing an egg cell.

This follicle produces more estrogen, which at around day twelve through day fourteen will cause a spike in the estrogen level to provoke a release of LH (luteinizing hormone) from the anterior pituitary gland (again, stimulated from higher up at the hypothalamus). The rise in LH causes the release of an egg from its follicle, called ovulation. Estrogen peaks near mid-cycle, which stimulates the production of LH (Luteinizing hormone). Ovulation is the egg release.

POST-OVULATORY

After ovulation, the remnant follicle still functions, releasing estrogen and, more importantly, progesterone, the hormone necessary to prepare the endometrium of the uterus for implantation and pregnancy.

The build-up of this uterine lining begins with the rise of estrogen during the follicular phase, and after ovulation, progesterone organizes and matures this tissue.

• If the egg is not fertilized, the progesterone level drops, and the thickened endometrium thereby loses its hormonal support, causing the tissue to fall apart around day twenty-eight. Unanchored to the uterus, the debris is seen as the bleeding known as menses (the menstrual "period"). The low hormones will again invoke the hypothalamic-pituitary-ovarian axis, the hypothalamus stimulating the pituitary to stimulate the ovary to kick-start the cycle anew.
   
• If pregnancy occurs, the estrogen and progesterone levels are maintained, and the hypothalamic-pituitary-ovarian axis is no longer in play.

 

The Lunar Month

The Menses (Latin, for "months"; singular, mensis), refers to the lunar month--28 days--and not the calendar months. 

(Pregnancy takes 9 calendar months, but 10 lunar months, due to the extra 2-3 days for each calendar month (except February).

Days 1-5, levels of estrogen are low and there is no progesterone. The endometrium is thin and without an epithelial layer during this (menstrual) phase.

Days 6-14, During Days 6 - 14 of the menstrual cycle, also known as the follicular phase, the levels of estrogen in the body increase. This rise in estrogen stimulates the endometrium, the inner lining of the uterus, causing it to thicken and develop an epithelial layer.

Day 14, ovulation, with some variation among women, occurs around Day 14.

Days 15-28, progesterone rises and matures the endometrium as arcuate (blood) vessels develop and enlarge. 

• An unfertilized egg will not attach to the endometrium. By day twenty-eight of the cycle, the endometrium is shed as bleeding, known as the menses. This cycle repeats until menopause, when follicular development ceases.
• If a fertilized egg does attach to the endometrium, the cycle ends by progressing into a constant state of estrogen and progesterone support for the ensuing pregnancy.

 

Abnormalities of Menstruation

• Dysmenorrhea: painful menses (period).
• Amenorrhea: absence of periods, usually associated with pregnancy, menopause, or hormone imbalance.
• Menorrhagia: heavy, painful menses.
• Hypermenorrhea: heavy bleeding with menses.
• Oligomenorrhea: menses that recur according to prolonged intervals (longer than 6 weeks), implying dysfunction of the whole cycle.