When a sperm penetrates an egg, the DNA from each parent combines to create a unique human being, known now as a zygote. This single cell begins to multiply immediately—from one to two to four to eight to sixteen cells, and so on.
As the cells multiply within the zygote, the earliest stage of the human embryo, it continues to grow. During this time, it is moving slowly through the fallopian tube toward the uterus, where it may implant into the uterine lining. If implantation occurs, the pregnancy will continue unless interrupted. The embryo will take approximately five days to reach the uterus, and it will be known as a blastocyst at this phase. The blastocyst is comprised of 70–100 cells. If implantation does not occur, the blastocyst will pass from the woman’s body during menstruation, resulting in an early pregnancy loss.
Essential Facts
• Under the right conditions and lighting, the human egg may be visible to the naked eye. The egg is about 25 times larger than a single sperm.
• Sperm, which are produced in the testicles, take about 70 days to reach maturity.
• The average number of sperm released during sexual intercourse ranges from 50 to 500 million.
• The tip of the sperm contains enzymes to break through the outer layer of the egg. Once the egg has received a single sperm, it immediately creates a barrier to prevent penetration by additional sperm.
• In addition to sperm, semen contains fructose, enzymes, citric acid, free amino acids, prostaglandin, potassium, and zinc.