Fertilization

To summarize, fertilization occurs when the sperm and the egg finally get together. Of the 100 million sperm ejaculated into the woman's vagina during sex, only a few hundred to a few thousand successfully make it into the two fallopian tubes. Many sperm are killed along the way by the hostile acid environment in the vagina or cervix. Still other sperm are simply not able to survive the long swim.

It is estimated that sperm may be able to live within the woman's fallopian tubes for up to three days. But remember that a woman's egg can only be fertilized within a specific 12- to 24-hour period. Therefore, pregnancy may occur during a time period of just two or three days each month. If any one sperm joins with a woman's egg, the process is called fertilization. When fertilization occurs, it almost always happens within the fallopian tube. The fertilized egg then travels through the fallopian tube and enters the uterus for implantation. (See Figure 3.4.)

These events must all methodically take place for a viable pregnancy to result. For example, both the egg and the sperm must be healthy. The body's hormones must be in proper balance. The lining of the uterus must be healthy and ready for implantation. If the fertilized egg implants anywhere but in the uterus, it is referred to as an ectopic pregnancy and must be detached. If any problem or obstacle arises within this essential chain of events, infertility issues may result. The chain of events looks like this:

• As the sperm travels through the woman's reproductive tract, it slowly releases enzymes from the sperm's acrosome.

• The sperm is now able to attack the cumulus granulose (outermost protective layer) and the zona pellucida (the shell-like covering) of the egg.

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