The normal human menstrual cycle should occur in 21- to 35-day intervals.  A particular woman usually has a certain cycle length that is rather constant for her that only varies 1 to 2 days per month. The fact that menstrual cycles occur regularly indicate that ovulation is occurring. When a young woman has her first menstrual period it is usually several years before she achieves cycle regularity.  Once she achieves cycle regularity she usually develops and ovulates an oocyte (egg) each month.

There is a finely tuned and highly coordinated system that orchestrates this process within the female reproductive system. The major control mechanism is the anterior pituitary which secretes two hormones (LH, FSH) in a pulse fashion every 60 to 90 minutes that presents a precise signal to the ovary for follicle recruitment and selection. The cells surrounding the oocyte in the ovary respond to these pituitary hormones and secrete their own hormones (estradiol).  While a group of follicles began to develop early in the cycle, it is around cycle day seven (first day of menses is cycle day one) that the follicle destined to ovulate has been selected. It develops further for about another 7 days before the egg actually leaves the follicle.

Once the egg leaves the follicle and ovulation has occurred then the ovary switches to producing another hormone (progesterone) that dominates the remainder of the menstrual cycle for the next 2 weeks. Follicle development takes anywhere from 1 to 3 weeks while the corpus luteum which secretes progesterone functions for a rather precise period of time (12 to 16 days).  Altogether this leads to a total menstrual cycle of 21 to 35 days in a woman having regular ovulatory cycles.

Women can sense whether their reproductive system is functioning in a normal manner by certain symptoms that occur periodically. When a woman is close to ovulation and a follicle is at maximum size it is not uncommon to have transient (12 to 24 hours) one-sided pelvic pain/discomfort that is associated with ovulation.  The term for this midcycle discomfort is called mittelschmerz. Frequently women experience transient subjective symptoms immediately before their menstrual cycle when they have ovulated. These symptoms could be bloating, breast tenderness, mood changes, cramps, and other symptoms. This set of symptoms is called premenstrual molimina and is usually repetitive and unique for each woman. 

Some women experience pain with their menstrual flow which is called dysmenorrhea.  Dysmenorrhea is more common in regular ovulatory cycles. If dysmenorrhea is constant, mild to moderate in infertility, and not getting worse each month then it is not considered to be pathologic. However, if menstrual pain starts later in life and gradually gets worse, month after month, year after year, then it needs to be brought to a physician's attention.

When the precise pituitary/ovarian system goes awry then women experience a loss of regular menstrual cycles.  Irregular menstrual cycles can occur in many different patterns:

1. Irregular menstrual periods that occur at unpredictable short and long intervals (e.g., every 2 weeks followed by 6 weeks). The term for menstrual cycles that occur in a random pattern is called oligomenorrhea.
   
2. In other instances there can be complete absence of menstrual cycles which is called amenorrhea. Certainly, amenorrhea occurs normally when a woman is pregnant but any other time it represents some abnormality of the reproductive system.

A whole host of disorders and conditions can cause menstrual irregularity and absence of menses. Pregnancy is a physiologic change in menstrual cyclicity.  Weight changes, (either weight gain or weight loss) can disrupt the menstrual cycle. Stress is a common condition that disrupts menstrual cyclicity; this can be either psychological stress or physical stress (e.g. high levels of exercise).  Abnormalities in other endocrine organs such as the thyroid or adrenal gland can upset the menstrual cycle as well. Finally, women experience a physiologic change in menstrual cycles (from regular to irregular cycles) as they age.  The average age of menopause is age 50 and a normal woman experiences 3 to 4 years of menstrual irregularity prior to the menopause.

A women's menstrual cycle is a good barometer for her general health. If there are any significant changes in her health her menstrual cycles are usually affected. As a rule of thumb if you have a distinct change from your normal pattern in menstrual cyclicity for more than 2 to 3 months then it would be wise to consult a physician.