Women everywhere share a common bond. We all experience a lifecycle of menstruation. For some, the journey is longer; for others, medical circumstances or choice shortens this cycle. But there are nonetheless certain stages that every woman experiences. The amount of fulfillment we get out of each stage is up to us.

A. Ovulation & Fertility

Without ovulation, there is no fertility. That's why birth control is so effective. Most birth-control pills prevent ovulation-that's how they work. But to fully understand the role ovulation plays in fertility, we must start with the basics.

B. Reproductive Anatomy: Internal Structure

Introduction

We believe that knowledge is a powerful tool. By giving you the straight facts about your body, She Comfort hopes to help you make healthy choices about your body-and your life.

In this section, you can find up-to-date and medically sound facts about the female reproductive system. Discover the parts of the reproductive system and how they work.

Keep in mind that while almost every woman's anatomy is basically the same, each person is also slightly different. Differences in size, color, and shape are completely normal. If you have concerns, contact your healthcare provider.

Internal Structure

We will start with these internal organs:

• Vagina
• Cervix
• Uterus
• Fallopian Tubes
• Ovaries

Vagina (Vuh-JEYE-nuh)

The vagina, also known as the birth canal, leads to the internal reproductive system. It is a narrow tunnel that measures between three and five inches in length. The vaginal opening is called the introitus. The vagina is surrounded by muscle and supporting tissue that can expand enough to allow passage of a baby. Natural vaginal secretions provide lubrication and keep a healthy balance of bacteria in the vagina to resist infection.

A certain degree of vaginal discharge is normal, and may change in consistency depending on the hormones present at different stages of the menstrual cycle. For example, during mid-cycle (ovulation), when estrogen levels are high, the cervix produces a large amount of watery secretions that you may perceive as vaginal discharge.

Normally, it looks clear and stretchy, and feels slippery-like mucus. Nature's aim during ovulation is to enhance reproduction: the mucus is abundant and slippery, giving sperm the best possible chance of surviving in the cervix. In contrast, cervical mucus becomes thicker or disappears entirely at other times of the menstrual cycle.

After menopause, and even the few years leading up to menopause and thereafter, vaginal discharge may decrease. This reduction in lubrication, which is caused by lower levels of estrogen, may cause dryness, irritation or even infection.

Cervix (SER-viks)

At the top of the vagina is the cervix, or the connection between the vagina and the uterus. The cervix itself is only about an inch in diameter, small, and pink. The opening to the cervix, called the os, is a very small hole in the middle of the cervix. After pregnancy, it appears as a 1/4-inch slit. The os opening is big enough to allow the flow of fluids, such as menstrual blood, from the uterus. During the labor of pregnancy, the os opens to nine-ten centimeters to allow delivery of the baby. In non-pregnant women, the os is only open a few millimeters. Cervical mucus acts as a barrier to bacteria by preventing bacteria from entering the uterus and the fallopian tubes.

Many women notice changes in the consistency of the cervical mucus during their cycle. It is often thin and mucus-like during ovulation when estrogen levels are high, and thicker and more sticky-or seemingly nonexistent-at any other time of the menstrual cycle.

Uterus (YEW-ter-us)

The uterus is a rose-hued, pear-shaped, muscular organ that can expand and stretch enough to accommodate the development of a fetus. The inner lining of the uterus is called the endometrium, which is a lining made up of blood vessels, specialized glands, and supporting tissue. This is the part of the uterus that is shed during a period.

There are three openings to the uterus. The cervix, which is the lower part of the uterus that opens to the vagina, and each fallopian tube which enters the uterus towards the top, one on either side.

The main function of the uterus is to create a nurturing environment for the growing fetus. During pregnancy, this small mass of muscle starts out about the size of a pear, and grows to become the largest muscle in the body, larger even than thigh muscles.

During menstruation, the uterus may contract in response to a series of hormonal changes: the shedding endometrium releases prostaglandins, which trigger contractions. Furthermore, the decline in progesterone that occurs just prior to menses may also contribute to uterine contractions. During labor, uterine contractions thin the lower segment of the uterus and cervix, a process called effacement, and expand the cervical os to prepare for delivery of the baby (dilation). Uterine contractions also assist in the actual delivery of the baby.

At menopause, as estrogen levels fall off, the endometrial lining no longer sheds and menstruation comes to an end.

Fallopian tubes

Besides the cervix, there are two other openings in the uterus leading to two fallopian tubes. These soft, limp tubes extend about five inches from the uterus to the ovaries. There are four components that make up the fallopian tubes. The first is the intramural component, which is the segment that goes through the uterine muscle. The second component is the isthmus, or the first part of the tube after exiting the uterus. The next component is the ampulla. This is where fertilization occurs. Finally, the fallopian tubes end at the fimbria, which are fringed and trumpet-shaped with minuscule feather-like tissue at the end which grasp eggs (ova) that are released from the ovaries.

From a reproductive standpoint, the fallopian tubes are designed to perform four related functions:

• Connect the ovary and the uterus
• Transport sperm in the right direction (from the fallopian tube toward the uterus)
• Provide a meeting place where conception happens
• Help propel the fertilized egg by producing gentle, continuous contractions that move the egg toward the uterus

Ovaries (OH-ver-eez)

The fallopian tubes lead to the ovaries which are oval-shaped organs that secrete hormones and house eggs, or ova. Measuring about an inch and a half wide and an inch long, the two ovaries sit on either side of the uterus, attached to the uterus by a ligament

The ovaries can be smooth, or during ovulation, they become marked by clusters of rounded bumps, or follicles, which house and nurture eggs. 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 womb, 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 uninterruptible. Only between four and five hundred will ripen into mature eggs during a lifetime.

During the first half of the menstrual cycle, the follicles are growing and secreting estrogen and the egg is undergoing the maturation process. The egg continues to grow until it is released from the follicle and picked up by the fimbria and transported to the fallopian tube. Meanwhile, the empty follicle cells coalesce into a yellow mass, called the corpus luteum, which secretes estrogen and progesterone.

Progesterone is produced to support the gestation (or nurturing) of an egg in the event that it is fertilized and implanted in the uterus. If pregnancy does not occur, the estrogen and progesterone secretion from the corpus luteum will cease 11-16 days after ovulation. Without the support of the hormones, the endometrium will shed. Over time, the corpus luteum becomes incorporated back into the ovarian tissue.

As menopause approaches, ovarian estrogen begins to decline. Estrogen levels become very low once there are no remaining follicles in the ovaries. Without ovarian production of estrogen and progesterone, the endometrium is not stimulated to grow and shed. This eventually leads to the end of menstruation.

The Ovarian Clock

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, 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 osteoporosis, and possibly heart disease and 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 which 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-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-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-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 hormone 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.

Puberty

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-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 hypothamo-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 1,000 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.

Menopause

Just as the early reproductive years are characterized by unpredictable menstruation, so are the years that precede the last menstrual period. 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.

Summary

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.