The Menstrual Cycle

by Marcelle Pick, OB/GYN NP

A woman’s menstrual cycle is highly individual, and can range anywhere from 24–37 days. What’s “normal” is what’s normal for you. The length of your cycle may be affected by many things, including illness, stress, travel, fertility medication, and close association with other women.

At different times in your life, your menstrual cycle may become irregular due to a variety of factors, including pregnancy, stress, diet, hormonal imbalance, exercise, and illness.  For more information, refer to our articles on irregular periods.

Let’s consider a typical 28–day cycle, and call the first day of bleeding day 1.

Menstrual Cycle Diagram

Menstrual Cycle Diagram – © 2014 Women to Women*

Each month our ovaries begin to ripen a number of follicles, which is why days 1–14 are called the follicular phase. Each of the many follicles in the two ovaries is a pocket of tissue filled mostly with estrogens. The number of active follicles changes with each cycle, but typically only one follicle per month in a single ovary becomes dominant over the others and produces a viable egg. As the follicles ripen, estrogen levels rise.

Just before mid-cycle, the hypothalamus and pituitary gland release LH (luteinizing hormone) and FSH (follicle stimulating hormone) spikes to trigger the dominant follicle to ovulate. Estrogen levels continue to rise.

Days 14–28 are termed the luteal phase, when estrogen levels begin to fall and progesterone levels rise. Around day 14 (but it can vary) ovulation occurs.  The dominant egg is released from the follicle and is drawn into the fallopian tube on its way to the uterus. This triggers a host of hormonal secretions — including estrogen — that thicken the uterine lining to support a pregnancy. Many women know they are ovulating by vaginal mucus discharge, nipple tenderness, or the tell-tale twinge in their abdomen called mittelschmerz.

Women are fertile for about 24–48 hours around the time of ovulation. Progesterone levels remain high unless the egg goes unfertilized, in which case the egg is re-absorbed and progesterone levels fall. In this event, progesterone levels continue to fall until day 28, when progesterone reaches its lowest level, menstruation occurs, and the cycle repeats. This turning point almost always occurs 14 days after ovulation.