A baby is created by the union of an egg cell from a woman’s body and a sperm cell from a man’s body. This union is called fertilization and marks the terming of pregnancy. Each egg and sperm cell contains half of the genetic material, or chromosomes, necessary to begin human life. These chromosomes contain thousands of sections called genes, which determine the various characteristics of the child. The woman and man each contribute twenty-three chromosomes to their child, making a total of forty-six. Because of the great number of possible combinations that can be produced by these forty-six chromosomes and their thousands of genes, every child is unique. The woman’s biological contribution begins in one of her two ovaries. Every month, an egg follicle ripens and swells in one of the ovaries. This ripening of the egg cell is initiated by a pituitary hormone. The walls of the womb surrounding the ripening egg produce estrogen, which causes the lining of the uterus, or womb, to thicken. The estrogen also causes the cervical to increase and to become more receptive to sperm. When the egg is attire, it bursts out of the follicle and is released near the fringed end of the fallopian tube. The release of an egg from an ovary is called ovulation. Ovulation usually occurs 14 days before the next menstrual period, or about midway through the cycle.

The fallopian tubes are muscular canals lined with fine hairs, called cilia, that move with a wavelike action, drawing the egg into the tube and then through it toward the uterus. At the same time that the egg is traveling through the fallopian tube, the follicle, stimulated by a pituitary hormone, begins producing progesterone, another hormone, which causes the uterine lining to thicken further. The progesterone also slows down contractions in the uterus, which facilitates implantation of the fertilized egg.

The biological contribution that the man makes to the baby begins with the production of sperm cells in his testes, which are two organs that hang outside his body in a sac of skin called the scrotum. Sperm cells, or spermatozoa, are produced in the seminiferous tubules within the testes and are propelled into the epididymis for storage until ejaculation. As the spermatozoa pass from the epididymis through the vas deferens to the urethra, secretions are added from the seminal vesicles, the prostate, and the Cowper’s glands. The purpose of these secretions is to provide a nourishing and fluid material that helps the spermatozoa move through the vagina, where they are deposited during intercourse.

The penis becomes erect during sexual excitement as blood pours into its layers of spongy material and the veins leaving the penis begin to constrict. With further excitement, the muscles around the seminal vesicles, the vas deferens, and the prostate gland contract, drive the semen into the urethra. The muscles in the penis then contract and push the semen through the opening of the urethra. About 250 million to 500 million sperm are ejaculated, although less than 500 will reach the egg.

For fertilization to occur, sperm must reach the egg within 24 hours of ovulation. Sperm remain viable for up to 72 hours. It takes sperm from 30 minutes to 2 hours to swim from the upper vagina, where they are usually deposited, to the outer third of the fallopian tube, where fertilization takes place. Sperm may reach an egg even if they are deposited externally, on the vulva. Only one sperm, out of the hundreds that may complete the journey, actually fertilizes the egg. As soon as this one sperm breaks through the egg wall, an enzyme is released that toughens the membrane of the egg and prevents penetration by any other sperm.