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Sperm are the tadpole-like cells that contain a man’s genetic heritage. Immature sperm are made in the testicles and then move slowly through a long, tightly coiled tube at the back of each testicle called the epididymis. When they leave the epididymis, sperm migrate up two thin pipes called the vas deferens. The vas deferens pass through the prostate gland and join the urethra, which is the tube that passes urine from the bladder.

A lot of guys think that their semen is made in the testicles. Not true. Sperm are made in the testicles, but semen—the liquid with which sperm are mixed during ejaculation—is made by the prostate gland and seminal vesicles.

In a healthy male, teaspoon of semen contains roughly 250 million sperm. Healthy men produce sperm at the prodigious rate of about 60,000 every minute. But each individual sperm cell takes about three months to grow.

Healthy sperm swim vigorously in a relatively straight line, their tails are long and whip-like, the heads are well-shaped, and the genetic information they carry is intact. Their swimming ability is key. Although it is only a matter of inches from the back of the vagina, where semen is normally deposited during unprotected sex, to an egg in a fallopian tube, sperm are so tiny that the journey is roughly equivalent to a man running three miles. Technically, a sperm’s swimming ability is called “motility.”

A sperm’s shape is called its “morphology.” Abnormally-shaped sperm often contain genetic errors of one kind or another, which reduce the chances the sperm will fertilize an egg.

The number, motility, and shape of sperm all generally decline with age, though many other factors can speed up this particular facet of a man’s overall sexual health. Heat, for example, is bad for sperm. Sperm production plummets in the days following a high fever, and anything that unnaturally warms the testicles, such as taking frequent soaks in a hot tub, will also hurt sperm production.

A final word of caution: research shows that a range of genetic problems in sperm get worse with age including defects that cause hemophilia, neurofibromatosis, Marfan Syndrome, and polycystic kidney disease. The risk of fathering a child with a birth defect rises with the age of the father, particularly when the father is older than 35, which is precisely when more and more couples are having children.

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