The prostate is a small gland approximately the size and shape of a walnut. It sits directly below the bladder, in front of the rectum. The prostate is a part of the male reproductive tract. It produces fluid that combines with sperm to make semen.
At birth, the prostate gland is tiny. When testosterone levels rise during puberty, the prostate grows rapidly, doubling in size by age 20. Growth slows down for the next two decades and the prostate usually does not cause problems for many years. Less than 10% of 30-year-old men have an enlarged prostate. When a man reaches his 40s, the prostate goes through a second growth spurt. Half of all men have an enlarged prostate by the time they reach age 60, and by age 85, 90% of men have an enlarged prostate.
The prostate surrounds the tube that carries urine from the bladder (urethra). During puberty, the prostate expands evenly. The enlargement that occurs in the second half of life, however, is concentrated in the part of the gland next to the urethra. As the gland gets bigger, it interferes with the flow of urine out of the bladder. That makes the bladder work harder to empty urine. Over time, the problem worsens, and eventually not all the urine can be emptied. The bladder wall also may become thickened, which can result in muscle spasms.