As long as a sperm cell is alive in the fallopian tube, it's capable of fertilizing an egg. If there's no egg in the fallopian tube, there's no chance of fertilization.
The fallopian tubes are about four inches (10 centimeters) long and transport the egg from the ovary to the uterus. They also provide sperm that make it that far with nutrients and a safe environment, the same kind the egg enjoys as it passes through. Of the millions of sperm cells that initially enter the cervix, there may be anywhere from one to a couple hundred that arrive at the fallopian tube [source: Regan].
Eggs will survive about a day after they're released from the ovaries. If not fertilized, they'll break down. It's only during this day that a woman can become pregnant, though it may be a result of a sexual encounter days earlier, since sperm can survive in the fallopian tubes for a few days.
When an egg does pass through, the sperm have receptors that allow them to smell the eggs, which are surrounded by cells releasing the sweet scent: progesterone. Sperm cells most definitely become very active when an egg is present. In fact, progesterone makes sperm become so active that they slough off layers of proteins. Both the surge in activity and the loss of proteins enables sperm to pierce the egg. This process is called capacitation. Once this occurs, the sperm only have a few hours to live. Only a few -- perhaps half a dozen or less -- sperm cells will ever share proximity with the egg [source: National Geographic].
How does the sperm cell actually penetrate the egg? The head of the sperm, once making contact with the exterior of the egg, will more or less "pop," releasing enzymes that allow it to cross through the barrier.
Once a sperm cell penetrates the exterior of the egg, fertilization occurs -- its DNA payload is delivered as the sperm is absorbed by the egg. The genetic blueprint of the child is now set in stone. Once a single sperm enters the egg, the egg's protective protein covering changes and doesn't allow other sperm to enter.
From one egg that has been fertilized by one sperm cell, encoded genetic information coupled with cell growth will eventually create an entire human being.
But first, as we'll learn in the next section, that fertilized egg must find an ideal location where it can undergo radical change.