The Female and the Eggs
Human conception is an elegantly simple and marvelously complex series of events. It is important to have a full understanding of the way things are supposed to work, especially if you are trying to conceive and starting to worry about your likelihood of success. A good working knowledge of normal events will make the steps that can be taken to evaluate your fertility make more sense.
Sperm are constantly being produced in the testicles of the adult male. This process begins at puberty and continues throughout the remainder of the man’s life. The formation of a mature sperm takes about 70 days and can be likened to a conveyor-belt type of process. The sperm begins as a very basic cell and is constantly modified over those 70 days to become a cell that not only contains the male genetic information, but also is capable of actively seeking out and fertilizing an egg.
At the time of intercourse and ejaculation, the sperm move from the testicles through the vas deferens, through the urethra, and are deposited in the vagina. The sperm almost immediately begin to travel into the cervix and through the cervical mucus. The majority of sperm that reach the cervical mucus do so within a matter of just a few minutes. They then continue on into the uterus and up into the fallopian tubes. This is a long and arduous journey and has been likened in terms of relative length to a human swimming the English Channel back and forth seven times. Amazingly, the sperm can complete this journey in just a few minutes.
Of the millions of sperm deposited in the vagina, only a few hundred survive and make it into the fallopian tube—true survival of the fittest. The vast majority of the sperm in the ejaculate does not reach the cervical mucus and are killed in the acidic environment of the vagina. The ejaculate consists not only of the sperm but also of several cc’s of fluid from the prostate, which nourishes and protects the sperm. Since only the sperm enter the cervix (and a very small percentage at that), it is normal for the majority of the ejaculate to leak out of the vagina following intercourse.
As opposed to the ongoing production of the sperm, new eggs are not formed within the ovaries. All of the eggs a woman will have, usually about two million, are present when she is born, and the number continues to decrease until, at the time of menopause, the egg supply is depleted.
During each cycle many eggs begin to develop. Through a process that takes two to three weeks, usually just one of the eggs will reach maturity while the rest undergo a process known as atresia, or degeneration, and are lost forever. When that one egg is mature, it is released from the ovary and basically sits on the surface of the ovary surrounded by its protective cells. The egg will be capable of being fertilized for only the next 12 to 24 hours at most. If not fertilized within that time, it is simply reabsorbed by the body.
Timing Is Everything
The fallopian tube is the tube-like structure that sits between the uterus and ovary. The fimbria are lush, finger-like projections on the end of the tube near the ovary. When a mature egg is ovulated, the fimbria must actively seek out this egg and pick it up from the surface of the ovary; the egg does not just fall into the tube.
Once the fimbria have picked up the egg, it is transported by little hair-like projections on the surfaces of the cells on the inside of the tube toward the uterus into the portion of the tube known as the ampulla. It is here that the sperm and egg meet and that fertilization will occur.
It doesn’t take just one sperm to fertilize an egg. There are many protective cells surrounding the egg, and many sperm are lost while actively removing these cells in order to gain access to the egg. Only after these cells have been removed and a path cleared can a single sperm penetrate the egg. Once a sperm penetrates the egg, the protective layer around the egg immediately undergoes changes that prevent any further sperm from entering the egg.
After fertilization, the zygote, or early embryo, remains in the tube for another three or four days. While in the tube, continued development occurs. When the embryo is transported through the isthmic portion of the tube and into the uterus, it is usually about 20 to 40 cells in size. The embryo “floats” in the uterus for an additional couple of days before attaching to the wall of the uterus, a process known as implantation.
There are very few days in the normal menstrual cycle during which a couple may conceive. While sperm may survive for several days in the reproductive tract of the female, the egg is only healthy and capable of being fertilized for 24 hours at most. After that time, it is simply reabsorbed by the body. The process of egg development and ovulation determines all the timing of this fertile period. There are, really, only a few days at most in any given menstrual cycle during which conception can occur.
A normal menstrual cycle is defined as being 28 to 30 days in length (from the start of one period to the start of the next). While cycles of shorter or longer length may or may not be normal, cycles of this length are most common. For purposes of discussion, Day 1 is defined as the first day of normal menstrual flow. During the next 12 to 14 days, the events that result in the ovulation of a mature egg occur. This is called the “follicular phase,” so called because the egg develops in a fluid-filled sac called the follicle. As the follicle (and the egg within it) develops, it produces hormones, the most important of which is estradiol (the primary form of estrogen).
Your Fertility: Understanding Natural Conception
Estradiol is responsible for stimulating other events to occur, all of which will result in optimal chances of the egg being fertilized and a pregnancy resulting. For example, estradiol is responsible for the changes in the cervical mucus. During much of the cycle, the cervical mucus is an impenetrable barrier, protecting the uterus and tubes from bacteria and anything else, including sperm. At about the time the egg is mature and ready to be released, the cervical mucus changes as a result of the ever-increasing amounts of estradiol being produced by the follicle. The mucus becomes thin and watery, and rather than preventing sperm from entering the uterus, it actually enhances this activity for a couple of days (but only right around the time of ovulation).
Estradiol is also responsible for the development of the lining of the uterus (the endometrium). All of these events occur in synchrony so that on day 14 of the ideal 28-day cycle, ovulation occurs, the cervical mucus is penetrable, and the lining of the uterus is getting ready to accept an implantation.
Intercourse must occur reasonably near the time of ovulation. Shortly after ovulation, the cervical mucus again becomes too thick for the sperm to penetrate; too long before ovulation, the sperm simply will not survive long enough to fertilize the egg. Having intercourse every other day around the time of ovulation is probably optimal. If, for example, a couple has intercourse every other day (days 10, 12, 14, and 16 in a 28-day cycle), they have done all they can do—at least in terms of enhancing their chances of pregnancy. More frequent intercourse does not further improve the chances of pregnancy as there begins to be a decrease in the number of sperm available if intercourse occurs more frequently than once every 48 hours.
It is possible to predict ovulation, and thereby time intercourse. Ovulation is preceded by a surge, or rapid increase, of a hormone called LH (luteinizing hormone). This surge can be detected through the use of an ovulation predictor, which is an over-the-counter test sold in any drug store. There are many ovulation predictors available, but they all function by detecting an increase in the amount of LH in the urine. They are really quite reliable and may be of value in timing intercourse. When the ovulation predictor changes, suggesting imminent ovulation, it is time to have intercourse.
When Should We Worry?
The process is obviously more complex than outlined here; these are, however, the basic steps in the process. The next question is “How well does this process normally work?” The answer is, “It depends.”
Human reproduction is a tremendously age-sensitive event. For a couple in their early twenties with no fertility problems, their fecundity, or chances of becoming pregnant in any given month, may be as high as 20 to 25 percent. This results in a probability of conception within one year of about 90 percent. For a couple in their forties with no known problems, monthly fecundity may be as low as one to two percent. These are important numbers to know. Numerous studies have demonstrated a significant decline in human fertility beginning at about age 30 and progressing rather quickly thereafter.
Many couples worry that it may be taking them too long to conceive. This is a natural concern. In this modern world, we have become increasingly used to controlling anything and everything, and to making things happen when we want. We can heat an entire meal in minutes or cross the country in hours. But we can’t make ourselves be pregnant when we want. Nature doesn’t work that way. Nature requires patience, and some couples have more patience than others.
The real question is this: “How long should a couple be patient before they begin to seek some help and evaluation?” The answer again is that “it depends.” There are very specific definitions for the duration of unprotected intercourse before a couple should be considered infertile. A couple that has never been pregnant before is defined as infertile if they have been attempting pregnancy for one year without success. If a couple has a previous pregnancy but now have been trying for six months to have another child, they too are infertile by definition.
These are, however, only definitions. They do not mean that any couple must wait a mandatory year before they begin to seek some evaluation and help. In fact, in many cases, this wait would be inappropriate. Women beyond the age of 35 certainly should not wait a year before beginning at least some preliminary testing. After age 40, a good case can be made for suggesting that a couple contact a physician as soon as they make the decision to attempt pregnancy.
As a guideline, when a couple becomes concerned about their ability to conceive, they should schedule some time with a physician and talk it over. Depending on the circumstances, it may be that some simple reassurance is all that is warranted. It is, however, inappropriate for any physician to tell a couple, “Just relax, it will happen.” If a couple is worried, their concerns need to be addressed. Equally inappropriate is the response, “You need to try for a year before we can do anything.” This simply is not true. While it may not be appropriate to become overly concerned and perform a lot of expensive and extensive testing, some simple evaluation may go a long way toward reassurance.
If a couple expresses fears and concerns, these emotions must be considered valid: To ignore or belittle them is not helpful or fair to anyone. So if you’re worried, find a physician with an interest in fertility and schedule some time to talk to her or him. Just talking to someone about your concerns can’t hurt. You may or may not decide to pursue further testing at that time, but at least you can make that decision in a more informed fashion.
John C. Jarrett II, M.D., is the co-author of The Fertility Guide: A Couples Handbook for When You Want to Have a Baby (More Than Anything Else).