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.