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How Ovulation Happens

Cycle Definitions

To really understand ovulation and the menstrual cycle—and especially to understand irregularities or changes to the cycle—it’s important to know the events that result in normal cyclic function. Let’s start with some facts and definitions. They’ll help you learn more about the process, and will give you the common language and vocabulary to discuss it with your doctor, friends, or even women’s health message boards.

Day 1: the first day of normal flow of the menstrual period. In an idealized 28-day menstrual cycle, ovulation occurs on day 14.

Follicular Phase: the phase of the menstrual cycle beginning on day one and continuing until the time of ovulation.

Proliferative Phase: describes the development of the lining of the uterus. It occurs during, and is synchronous with, the follicular phase.

Luteal Phase: the phase of the cycle beginning at the time of ovulation and continuing until the next menses begin.

Secretory Phase: describes the changes that occur in the lining of the uterus after ovulation. It occurs during, and is synchronous with, the luteal phase.

Follicle: the fluid-filled sac that contains the developing egg and the cells surrounding that sac.

Estradiol: the primary estrogen produced by the developing follicle.

Corpus Luteum: after release of the egg, the follicle becomes the corpus luteum and begins to produce progesterone.

Progesterone: the principal hormone produced by the corpus luteum.

How the Ovaries Work

At the time of puberty, the ovaries begin to function in a cyclic manner that results in menstrual cycles, which results in the release of one egg per month. The hormones associated with the development of that egg prime the rest of the reproductive system to allow, and then accept, the hopeful pregnancy. This cyclic functioning continues until menopause, at which time the development and release of eggs, and the production of the associated hormones, ceases.

The Follicular Phase

The ovarian cycle can be divided into two phases:

  1. The follicular phase
  2. The luteal phase

It is during the follicular phase that an egg begins to develop and mature inside a fluid-filled sac within the ovary called the follicle. During its development, the follicle will increase in size from a microscopic dot to two centimeters or more at the time of ovulation.

Although only one egg is typically released each month, many eggs actually begin to develop. This development begins near the end of the previous menstrual cycle, even before the period has started. As these eggs continue to develop, fewer and fewer of them progress until only one remains. The remainder of the eggs that don’t develop undergo a process known as atresia and are lost forever. The one egg that is destined to ovulate is usually selected as early as day five or six of the menstrual cycle. This follicle then continues to develop while the rest of the ones that had started to develop regress.

The follicle in which this egg develops is primarily responsible for the production of estradiol, the principal hormone responsible for the development of the lining of the uterus, the changes in the cervical mucus, and so on.

Egg and follicle development in the ovaries are under the control of a pituitary hormone known as follicle-stimulating hormone (FSH). Early in the follicular phase, FSH is produced by the pituitary in relatively large amounts, and this signals the eggs and their follicles to begin to develop. FSH continues to control the development of the egg(s) and follicle(s) throughout the follicular phase. When the one egg destined to ovulate has reached maturity and is ready to be ovulated, a second pituitary hormone known as luteinizing hormone (LH) triggers the release of the egg from the ovary. FSH primarily stimulates development and maturation of the follicles and eggs. LH primarily stimulates ovulation.

The Luteal Phase

Once the egg has been released, the cells of the follicle remaining in the ovary quickly change and become known as the corpus luteum. These cells then begin to produce significant amounts of progesterone in addition to estradiol. Progesterone induces the cells of the lining of the uterus to undergo the changes that will allow an embryo to implant and begin to grow.

The life span of the corpus luteum is limited. Unless a pregnancy occurs and continues to stimulate it, the corpus luteum will produce progesterone for only 12 to 14 days. As the corpus luteum begins to fail and progesterone production falls low enough, the cells of the lining of the uterus will begin to shed, a menstrual period begins, and the whole process starts over.

The life span of the corpus luteum is limited. Unless a pregnancy occurs and continues to stimulate it, the corpus luteum will produce progesterone for only 12 to 14 days. As the corpus luteum begins to fail and progesterone production falls low enough, the cells of the lining of the uterus will begin to shed, a menstrual period begins, and the whole process starts over.

Of course, not all menstrual cycles are exactly 28 days long. Cycles from 26 to 30 days or even longer may be normal. Normal ovulation occurs 14 days before the onset of the next menstrual period. Therefore, in a 32-day cycle, ovulation probably occurs on or about day 18.

If a woman has regular menstrual cycles, she is in all probability ovulating. However, there are instances in which bleeding can occur in the absence of ovulation, but this bleeding is typically very irregular and often abnormal in amount or duration.

Ovarian Reserves

It is thought that a woman is born with all the eggs she will ever have—no new eggs are ever produced. With each menstrual cycle, some of these eggs are used up. There are always fewer and fewer eggs left in the ovary for each subsequent cycle. Furthermore, the healthiest and most responsive eggs are probably the first ones to be ovulated. In other words, when a woman is 20, the eggs being released at that time are very fertile. By the time she reaches 40, the eggs remaining have been through as many as 350 cycles without ovulating—they just aren’t as responsive to the stimulation from the pituitary hormones and have less fertility potential. With aging, the functioning of the ovaries declines and the capacity of the remaining eggs to establish a normal pregnancy decreases.

Almost all of the decrease in female fertility with aging is due to this fact. This is known as ovarian reserves. How many functional eggs remain, and determination of ovarian reserves, is often important, particularly in older individuals.

Getting some idea of the extent of an individual’s ovarian reserves can be a very important step in providing a couple with some idea about their chances for successful conception, especially before initiating expensive and time-consuming treatments. Both a day-three FSH level and a clomiphene citrate challenge test give us this information. After age 30, and certainly after age 35, these tests should be considered to be sure the ovaries are capable of producing adequate eggs, both in quantity and quality, before initiating some of the more expensive treatment alternatives.

How You Can Strengthen Ovarian Function

There are a couple of things every woman can do to help ensure that her ovaries work to their maximal potential. First of all, don’t smoke. There is excellent evidence that cigarette smoking not only makes it harder to get pregnant, but it also has a very deleterious effect on ovarian reserves. Women who smoke typically have far fewer functional eggs remaining in their ovaries than does someone their age who does not smoke.

Secondly, watch your caffeine intake. Consuming more than the equivalent of a couple of cups of coffee per day has a negative effect on your chances of conceiving.

Finally, if you are even considering getting pregnant, it is a good idea to take a prenatal or multivitamin. While this may not affect your chances of conceiving, taking a prenatal vitamin does help to decrease your risk of birth defects such as neural tube defects.

Exercise and Ovulation

If you exercise on a regular basis, should you continue to do so if you are trying to get pregnant? For most individuals, the answer is yes. Unless a woman exercises to an excessive level, there is no adverse effect of this activity. Women who train for and run marathons, or other elite athletes, may indeed experience changes in their menstrual cycles that make successful conception difficult. If, however, you exercise regularly to stay in shape, continue to do so. It won’t hurt your chances of getting pregnant and can be an excellent way to help you deal with the stress that the difficulty of getting pregnant may pose.

Weight and Ovulation

It is important to maintain your body weight in a normal range. Women who are too thin have a much higher incidence of ovulation problems. These can range from luteal phase defects to total absence of ovulation. By the same token, being excessively overweight can significantly interfere with normal egg production and ovulation.

Try to maintain a positive attitude. There is no question this can be very difficult if conception does not come readily. Remember, the treatment of impaired fertility is more successful than ever before, and the vast majority of couples can and will be successful in their pursuit. A positive attitude helps! There is good evidence that women who learn self-relaxation techniques or positive imaging techniques do better, and have higher success rates, than women who do not.

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