Menstruation 101: The Menstrual Phases

Menstruation 101: The Menstrual Phases

The Menstrual Phase

On Day 1, the menstrual period begins. Every month a woman’'s body builds up a fresh lining of tissue in the uterus in response to increasing amounts of the reproductive hormones estrogen and progesterone. If a woman is not pregnant, the levels of these two hormones decline. When these hormone levels have fallen to their lowest point, the lining, which is known as the endometrium, is shed from the uterus, through the cervix and, vagina and out the vaginal opening. An average period lasts between 3 and 7 days.

The Pre-Ovulatory (Follicular) Phase

As estrogen levels begin to fall during the few days prior to the onset of menstrual flow, an increase of follicle-stimulating hormone (FSH) is triggered. This, in turn, stimulates the growth of ovarian follicles. Each follicle contains an egg. Every month, FSH initiates and promotes the growth of a group of follicles. Soon, one or two follicles assume dominance, continuing to grow, mature, and produce estrogen. The lining of the uterus begins to thicken in response to this increase in estrogen. Estrogen levels rise dramatically during the few days preceding ovulation. This is a signal that a mature follicle is ready to ovulate. The high levels of estrogen trigger the luteinizing hormone, or LH, surge from the pituitary gland.

Over the course of a lifetime, a woman releases about 500 eggs in their mature form. The number of eggs that are contained in the ovaries depends on the age of the woman. The highest number is actually found before a girl is born. While still in the mother’s uterus, a 20-week-old female fetus has approximately seven million eggs. At birth, the number has decreased to two million. By the time a girl enters puberty, she has between 300,000 and 500,000 eggs. This decline in number is the process called atresia, a natural and continuous process that is uninterruptable. Only between 400 and 500 will ripen into mature eggs during a lifetime.

The Ovulation Phase

The increase in luteinizing hormone (LH) triggers ovulation. Ovulation is an event that occurs when a mature egg is released from an ovarian follicle to the nearest fallopian tube. Fertilization, the union of an egg and sperm, occurs in the fallopian tube. The egg, fertilized or not, then travels into the uterus.

At ovulation, in addition to estrogen, progesterone is now produced. These two hormones combined stimulate the dramatic changes in the endometrium that are necessary to allow a pregnancy to implant and grow. In a woman who has regular 28 day cycles, ovulation usually occurs on day 14. However, most women have different cycle lengths. In general, ovulation occurs 11-16 days before your upcoming period.

The Premenstrual (Luteal) Phase

After ovulation, the follicle becomes the corpus luteum. The cells of the corpus luteum produce estrogen and progesterone and these two hormones are involved in the development of the uterine lining. If a woman does not become pregnant, the corpus luteum regresses about two weeks after ovulation. Because of this, estrogen and progesterone levels drop and the stimulation for the endometrium is lost. This causes the shedding of the lining to begin, and a new menstrual cycle commences.

If a woman becomes pregnant during a menstrual cycle, fertilization will occur within 24 hours after ovulation. Five days after fertilization, the embryo (fertilized egg) enters the uterus and becomes embedded in the endometrium a few days later. With implantation, cells that will eventually become the placenta begin to produce human chorionic gonadotropin (HCG), which is sometimes called the“pregnancy hormone.” HCG interrupts the menstrual cycle by providing continual stimulation of the corpus luteum to produce estrogen and progesterone. This prevents the loss of the endometrium.

Menstruation: Charting Your Cycle

Changing hormone levels in the menstrual cycle bring a variety of bodily changes each month. There are changes in vaginal discharge, increases and decreases in body temperature, and abdominal twinges and pain. Charting the menstrual cycle over time will allow a woman to predict her menstrual periods, note changes in her body, and track possible premenstrual symptoms. Charting the cycle also gives a woman more control over her reproductive health and allows her to appreciate her own unique bodily rhythms.

Keeping track of the monthly cycle is simple, but it does require diligence. We have provided an interactive calendar on this site which you may want to use to record your cycle. Or, if you prefer, use a calendar of your own. Be sure to note the first day of your period. Also consider charting episodes of cramps, spotting, or any other important symptoms. Pretty soon, a cyclical pattern may emerge and you may be able to relate certain symptoms to your menstrual function.

To further identify times when symptoms occur, you may want to consider charting your basal body temperature. Most gynecologists have a graph that can be used to record changes in temperature. A digital thermometer is easy to use and the temperature should be taken first thing in the morning, after a good night’s sleep and before getting out of bed. You will notice an increase in basal body temperature once ovulation has occurred. This increase ranges from one half to one degree Fahrenheit. If pregnancy does not occur, your body temperature will return to its pre-ovulatory level as estrogen and progesterone levels drop and you approach your next period. If, however, you are pregnant, basal body temperature will remain elevated. If your temperature remains elevated longer than 16 days from ovulation, you may be pregnant.

The Ovarian Clock, By Dr. Cristina Matera M.D.

Our lives are composed of many stages, some of which are subtle or blurred transitions such as the passage from childhood, through adolescence, and into adulthood. Others are defined by rather specific events, for example, when the pre-school years are abruptly ended by the first day of school and when graduation catapults us into the responsibilities of employment and professionalism. Many women also categorize their lives by their“gynecologic age” or reproductive capabilities. The profound transition from an asexual child, to an energetic young woman, to becoming a mother, and finally to a mature, experienced woman in the post-reproductive or menopausal years can be viewed as gradual, but is also punctuated by a distinct event— the first and the last period.

Unlike men, women receive an undeniable signal informing them of their fertility (their first menses) and are also given a“fixed dose” of fertility which begins declining in the mid-30s and completely ceases at menopause in the late 40s to early 50s. In addition, the female hormone, estrogen, is known not only to be crucial in reproduction, but also to protect women from certain ailments such as heart disease, osteoporosis, and possibly Alzheimer’s disease. In contrast, the high levels of testosterone found in men may actually predispose them to conditions such as heart disease and prostate cancer.

The changes in women’s bodies that accompany the transition from the pre- to post-reproductive years are governed by the normal development, function, and eventual senescence of the ovaries. Most people do not spend a lot of time dwelling on these small, walnut-sized organs that are so vital to our health, livelihood, and future generations; however, since the ovarian life cycle is such an important aspect of each and every woman’s life, it may be of interest to understand how the ovaries are formed in a female embryo and work in a female body.

Development: The formation of the ovary

The two major roles of the ovary are to produce a mature egg for ovulation each month and to secrete the reproductive hormones. The functional unit within the ovary is the follicle. This structure is composed of an egg and the cells that surround the egg that manufacture estrogen, progesterone, and testosterone (albeit in much smaller quantities than in men).

The formation of the ovary begins very early in a developing embryo during pregnancy. The precursor cells, eggs in women and sperm in men, referred to as germ cells, are actually first found outside of the embryo in a structure known as the yolk sac and at five to six weeks of pregnancy they migrate from the yolk sac into the area that will eventually become the gonad (the generic term for an ovary in a woman and a testicle in a man). If the germ cells do not successfully migrate and arrive to this region, an ovary or testicle will not develop and only scar tissue will form. At six weeks of pregnancy, this gonad is considered“bipotential”, or can develop into either an ovary or a testicle. If that embryo has the genetic make-up of a boy (46, XY), the gonad will receive the necessary information to transform into testes. If the“testes determining factor” is lacking, as in a genetic female (46, XX), an ovary will develop instead. This process occurs between six to nine weeks of pregnancy.

One of the first signs that the gonad will be an ovary is the exponential increase in the number of eggs, which occurs as a result of the duplication of the chromosomal content and the subsequent division of one egg to make two eggs. By the fifth month of pregnancy, the female fetus contains six to seven million eggs, the maximum number that will ever be reached. Equally astounding is that over the course of the next four months of the pregnancy, there is such a rapid decline in the number of eggs that at birth only two million eggs remain. Why and how a woman loses 80% of her initial endowment of eggs prior to even being born is not completely understood. The vast number of eggs are lost in a process called“atresia”, where the follicles enter a growth phase that is never ultimately completed. This results in the demise of the egg and the other cells of the follicle get reincorporated into the ovarian tissue. The process of atresia begins in the female embryo and continues uninterrupted throughout a woman's lifetime; however, at no other time will it occur at such a rapid rate as prior to birth. For every one egg that is“lost” by ovulation, thousands are lost in the process of atresia. This limited or finite number of eggs contrast sharply with what occurs in men, where sperm are constantly being regenerated. Although you will see waves of follicular growth in the ovary of the embryo, full maturation, ovulation, and significant estrogen production do not occur. The situation differs in male fetuses where large amounts of testosterone are produced and are critical for the development of normal male genital organs.

The ovary in childhood

Soon after birth, when the baby girl is separated from the placenta (the source of huge amounts of estrogen and progesterone), the circulating levels of these hormones abruptly plummet. This results in a rebound or burst of activity in the ovary with multiple follicular growth. Eventually and often by the first birthday, the ovaries enter a quiescent stage. Although continuing waves of follicular growth occur, all of these follicles are doomed to fail and undergo atresia. There is essentially no hormonal production from the ovaries in the childhood years.


When a young woman reaches puberty, only 300,000 - 500,000 eggs remain. In a woman’s lifetime, 400 to 500 eggs will be released from the ovary by ovulation. The subtle and perhaps not-so-subtle signs that herald the onset of puberty in girls are due to the rise in the production of estrogen and testosterone. Sexual hair growth (pubic and underarm) results from male hormone production from the ovaries and the adrenal glands (two small organs found above the kidneys). Estrogen is responsible for breast development and the rapid growth seen in early adolescence. The rising estrogen levels also stimulate the growth of the lining of the uterus (the endometrium). The eventual sloughing of this lining is a woman’s first period. By the time that a woman begins to menstruate, she has practically completed the pubertal transition.

The reproductive years

Rarely does a young woman ovulate during the first few menstrual cycles. In fact, the irregular menses in the first one to two years is typical until the ovulatory process is fully functional and mature. This maturation process occurs in a specific area of the brain (the hypothalamus and the pituitary gland). Different hormones that are secreted from these glands stimulate the ovary and when the dialogue of the hypothalamo-pituitary-ovarian axis is organized, regular and monthly periods will occur. As the follicle is growing and maturing in preparation for ovulation, an increasing amount of estrogen is being produced. Progesterone is only made after ovulation by the corpus luteum, the“ruptured follicle”. If a woman does not become pregnant the corpus luteum will cease to function approximately two weeks after ovulation. The decline in estrogen and progesterone levels removes the support of the now thickened endometrium and it will be sloughed as a menstrual flow. If a woman does conceive, the pregnancy hormone, hCG, will urge the corpus luteum to remain alive and make the hormones that are necessary to sustain an early pregnancy.

For each follicle that eventually ovulates, close to 1000 will have a limited but unsuccessful growth. The number of eggs that are lost per ovulatory cycle probably varies throughout a woman's life, but is presumed to be accelerated in the 10 - 15 years that precede menopause. At this time, not only are the numbers rapidly dwindling, but there is also a decline in the quality of the follicles. Hormonal production is not as predictable and robust and the eggs are also known to contain changes in their genetic make-up. The decline in the quantity and the quality of eggs during this time adequately explains the diminishing in fertility that is seen in women from age 35 and onward— the proverbial“biologic clock”. Even if a woman should conceive, her chances of having a miscarriage because of a baby with a resultant chromosomal abnormality increases greatly as she gets older.


Just as the early reproductive years are characterized by unpredictable menstruation, so are the years that precede menopause. Erratic follicular growth without ovulation occurs, but estrogen is still being made. When all of the follicles in the ovary have been depleted, estrogen production ceases and a woman now enters menopause. Nonetheless, the ovary does continue to manufacture male hormones. Although it remains controversial, these hormones are considered to play a role in maintaining bone health, normal libido or sex drive, cognitive function, and overall sense of well-being.


Thus, the story of the ovary begins way before a little girl is even born and ceases to function by approximately 50 years of age. This contrasts markedly from men who continue to have normal testosterone secretion and sperm production well into their 70s. The depletion of eggs may protect women from getting pregnant at a time when she may not have the energy or longevity to satisfactorily raise a child. Alternatively, it places pressures on the modern 20th century woman who is trying to complete her education, establish a rewarding profession, become financially secure, and also have a happy and close-knit family. The other concern is the loss of estrogen which predisposes women to various illnesses such as osteoporosis, heart disease and Alzheimer’s disease. Now that women commonly live into their 80s, this long term estrogen deprivation needs to be addressed to prevent disease and enhance the quality of life. With the information presented here, women are now empowered with knowledge that can help them make more well-informed decisions regarding important reproductive and health choices throughout their lives.

By Dr. Cristina Matera, M.D.

  • Period Calender
  • New to Tampons?
  • Try our slimmest applicators