Most women have approximately one quarter of a million egg cells, which are kept safe inside her ovaries throughout her lifetime. Midway through a woman’s menstrual cycle, a single egg is released from a fluid-filled sac called a “follicle.” At this point the egg, which is one of the largest cells in the body, is no bigger than a grain of sand. A single sperm must fertilize the egg within 24 hours or it will be absorbed, and the woman’s cycle will begin again.
Sperm must overcome a variety of barriers to successfully fertilize the egg. Out of the 200 to 300 million sperm ejaculated, only a small number of sperm will actually reach the egg. Twenty-five percent of the sperm die almost immediately upon contact with the acidic environment of the woman’s vagina. This acidic environment protects the woman from infection and bacteria.
The sperm’s only purpose is to fertilize an egg. They instinctively swim against the downward current of the fallopian tubes toward the egg.
One sperm can fertilize the egg, after which a biochemical reaction helps prevent any other sperm from getting inside. Once inside, the sperm loses its tail and contributes 23 of the 46 chromosomes necessary for human life. About 20 hours after fertilization, the new cell splits and is now called a “zygote.” The cells, each rapidly dividing, have little time for individual growth, which causes each new cell to be a little smaller than the previous cell. As the cluster of cells continues to divide, it becomes a hollow ball of fluid-filled cells, now called a blastocyst. The blastocyst floats down and implants itself in the nutrient-rich lining of the uterus to begin receiving nutrients from the mother. By two weeks, the blastocyst has grown to one tenth of an inch in length. The spine is developing, and the chambers of the heart are beginning to form. Soon the heart will begin to beat.