What Are the 'Normal' Chances of Getting Pregnant?

Excerpted from
Dr. Richard Marrs' Fertility Book : America's Leading Infertility Expert Tells You Everything You Need to Know About Getting Pregnant
By Richard Marrs, M.D.

To begin with, it might be of some comfort for you to realize that reproduction in even the most fertile human is inefficient compared with other animal species. If you really want to get pregnant, in your next life come back as a mouse, a cat, or even a fish. Lower species have all the luck. As a rule they ovulate multiple eggs, and their reproductive efficiency is extremely high. In fact, almost every time the females of lower mammalian species ovulate and have sexual activity, pregnancy results. One of the most common treatments for human infertility-using supraovulation therapy to produce multiple eggs during a single cycle-is an attempt to turn our evolutionary clocks back to what lower species do on a regular basis.

Normal Fecundity Rates

15 years old: 40-50% per cycle
25 years old: 30-35% per cycle
35 years old: 15-20% per cycle
45 years old: 3-5% per cycle

Unfortunately for us, human beings and rhesus monkeys (our closest evolutionary counterparts) are the two primate species designed to be reproductively inefficient. Why we were singled out to evolve to this level of reproductive inefficiency, no one really knows. But in our case ovulation occurs just once each month, with a single egg, and then an incredible series of variables must all fall into place for a pregnancy to ensue. Because of this, the real expectation for pregnancy to occur in any given cycle, even under the best of circumstances with a fertile couple whose timing is perfect, is, on the average, just 25% per month. This percentage rate is called fecundity-the ability to conceive during any given cycle of ovulation-and it varies greatly according to a woman's age.

These fecundity rates assume that you are ovulating normally, and that you are having appropriately timed sex. As you can see, all women, even women who are fertile throughout their lives, have reduced fecundity rates as they age. This declining ability to conceive can't be changed by exercise, vitamins, or attitude, because it's totally related to the age of your eggs.

Not too many couples understand that a woman's body produces all the eggs she will ever have by the time she's a six-month-old fetus, still in her mother's womb. Your eggs are present at your birth, and they're exposed to every toxin and environmental condition that you encounter as you go through your life. Because they contain such sensitive genetic packages, this exposure may greatly affect them; as time goes on, your eggs become progressively less efficient at producing a fertilized embryo that can become a healthy baby. This breakdown of eggs as they age is one of the biggest problems women face in infertility treatment today, because there's simply no way to reverse it.

And fecundity is just one of the hundreds of variables that affect your ability to get pregnant. Of course, what makes things even more difficult is that a woman's reproductive tract, unlike a man's, isn't visible. The vagina, cervix, uterus, fallopian tubes, and ovaries are all internal, which makes it hard to observe and comprehend the many cyclical changes that occur during your reproductive cycle.

Some Things Are Decided Even Before You're Born

Ovarian formation occurs between the fourth and fifth months of fetal life. Not only do the ovaries structurally form at this time, all the eggs you will ever have are made as well-on average, five to seven million.

Though this seems like a huge supply, you begin losing these eggs even as a newborn infant, more than a decade before your first ovulatory cycle Throughout these early years, a girl's follicles degenerate in a rapid fashion While they are not actually ovulated, this constant, day-today attrition uses up three million to five million eggs by the time you approach puberty and your first menstrual period (menarche) We don't completely understand why this happens, but we theorize that it may be due to a lack of certain protecting hormones, or to an inadequate level of the hormones that control follicle and egg maturity, such as follicle-stimulating hormone (FSH) and luteinizing hormone (LH). These hormones are produced by the pituitary gland, which is part of your brain. FSH and LH control the growth and development of the follicle and egg during the ovulatory cycle.

Whatever the reason, the net result is that most young women start menstruating between the ages of twelve and fourteen with just a half million to two million eggs remaining in their ovaries The exact number depends upon the individual, and unfortunately we have no way of determining exactly what it is in your case (or in the case of any woman), either at the beginning of your reproductive years or as you grow older. Because we don't know the number of eggs that are actually available, it is hard to predict how many will mature if you are medically stimulated in preparation for in vitro procedures This is a definite handicap when advising older women as to their possible success rate when they are considering infertility treatment.

How Your Reproductive System Matures

What we do know is that at or about the time of puberty, a specific part of your brain called the hypothalamus matures hormonally. This event is the catalyst that prepares the way for the beginning of ovulatory cycles and menstruation.

Within the human body, hormonal interaction and glandular function are cascading events that begin in the center of the brain, flow from gland to gland through a series of hormonal messages, and ultimately end up at a "target organ." During your ovulatory cycle the target organ is the ovary. For you, the process begins when your hypothalamus releases a hormone called gonadotropin-releasing hormone (GnRH), which signals the pituitary to release FSH and LH.

In women, FSH and LH target the ovaries. As your ovaries receive these hormones they, or more precisely the follicles within them, respond. As the follicles respond, they make estrogens, which stimulate the maturation of your body. Regular ovulatory cycles usually begin within two to six months after your first menstrual period. At that point the brain and ovary will have achieved a balance, and you will start to ovulate and menstruate on a regular basis.

How Your Ovulatory Cycle Works

It's during your ovulatory cycle that an egg develops within the ovary until it's mature enough to be released and enter the fallopian tube. There it may come in contact with sperm, and fertilization can take place. Because of its many complex stages, ovulation causes some of the most common infertility problems, and therefore is an area where couples will most often receive initial evaluation.

Conversations between the Brain and Ovaries

Ovulation requires an extremely delicate balance between the ovary and the central nervous system. This connection isn't physical; rather, your brain and your ovaries talk to each other through a series of hormonal releases that act as signals which when correctly balanced, produce normal ovulation. But even balanced hormonal releases will fail unless your ovaries are correctly formed and structured. This ovarian makeup is developed while you are still a fetus in your mother's womb.

Since 40% of all infertility problems involve the female there's a good chance that your treatment will include correcting an ovulatory disorder, attempting to improve ovulation, or using various medications to supraovulate (produce multiple eggs in an already ovulating woman). To understand why your doctor uses certain methods and medications to accomplish these goals, you must develop a basic knowledge of how your ovulatory cycle works, as well as some understanding of the roles of the various hormones that control it. For a moment, visualize yourself as a microscopic organism in the bloodstream, and follow the path of the hormones that control ovulation and menstruation.