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Physical Development in Boys: What to Expect

Enlargement of The Testicles and Scrotum

A near doubling in the size of the testicles and the scrotal sac announces the advent of puberty. As the testicles continue to grow, the skin of the scrotum darkens, enlarges, thins, hangs down from the body and becomes dotted with tiny bumps. These are hair follicles. In most boys, one testicle (usually the left) hangs lower than the other.

Pubic Hair (Pubarche)

Fueled by testosterone, the next changes of puberty come in quick succession. A few light-colored downy hairs materialize at the base of the penis. As with girls, the pubic hair soon turns darker, curlier and coarser in texture, but the pattern is more diamond-shaped than triangular. Over the next few years it covers the pubic region, then spreads toward the thighs. A thin line of hair also travels up to the navel. Roughly two years after the appearance of pubic hair, sparse hair begins to sprout on a boy’s face, legs, arms and underarms, and later the chest.

Changing Body Shape

A girl’s physical strength virtually equals a boy’s until middle adolescence, when the difference between them widens appreciably. During early puberty, both sexes add some fat, lending many boys a chubby appearance. The growth spurt soon offsets that; in fact, the dramatic increase in height often makes them look gangly. Boys continue to fill out with muscle mass long after girls do, so that by the late teens a boy’s body composition is only 12 percent fat, less than half that of the average girl’s.

Penis Growth

A boy may have adult-size genitals as early as age thirteen or as late as eighteen. First the penis grows in length, then in width. Teenage males seem to spend an inordinate amount of time inspecting their penis and covertly (or overtly) comparing themselves to other boys. Their number-one concern? No contest: size.

“Take one hundred boys, and they all feel like their penis isn’t as big as it should be,” says Dr. Mark Scott Smith, chief of adolescent services at Children’s Hospital in Seattle. He recalls examining one young patient who fidgeted anxiously throughout the office visit. When Dr. Smith asked him if he had any concerns or questions, the boy looked down at his lap.

Well . . .
Dr. Smith knew to ask, “Are you worried about your penis?”
Um, yeah.”The boy braced for the physician to confirm his fears that his penis was microscopic in size. No: the smallest penis in the history of humankind.

“I always say the same thing in that situation,” says Dr. Smith. “I told him, ‘I examine a lot of kids, and I can tell you that you are entirely normal.’ That seemed to reassure him.”

Most boys don’t realize that sexual function is not dependent on penis size or that the dimensions of the flaccid penis don’t necessarily indicate how large it is when erect. Parents can spare their sons needless distress by anticipating these concerns rather than waiting for them to say anything, since that question is always there regardless of whether it is articulated. In the course of a conversation, you might muse aloud, “You know, many boys your age worry that their penis is too small. That almost never turns out to be the case.” Consider asking your son’s pediatrician to reinforce this point at his next checkup. A doctor’s reassurance that a teenager is “all right” sometimes carries more weight than a parent’s.

Boys’ preoccupation with their penis probably won’t end there. They may notice that some of the other guys in gym have a foreskin and they do not, or vice-versa, and might come to you with questions about why they were or weren’t circumcised. You can explain that the procedure is performed due to custom, parents’ choice or religious beliefs.

“What Are These Bumps On My Penis?”

About one in three adolescent boys have penile pink pearly papules on their penis: pimplelike lesions around the crown, or corona. Although the tiny bumps are harmless, a teenager may fear he’s picked up a form of venereal disease. The appropriate course of action is none at all. Though usually permanent, the papules are barely noticeable.

Fertility (Spermarche)

Boys are considered capable of procreation upon their first ejaculation, which occurs about one year after the testicles begin to enlarge. The testicles now produce sperm in addition to testosterone, while the prostate, the two seminal vesicles and another pair of glands (called Cowper’s glands) secrete fluids that combine with the sperm to form semen. Each ejaculation, amounting to about one teaspoonful of semen, contains 200 million to 500 million sperm.

Nocturnal Emissions and Involuntary Erections

Most boys have stroked or rubbed their penises for pleasure long before they’re able to achieve orgasm—in some instances, as far back as infancy. A youngster may consciously masturbate himself to his first ejaculation. Or this pivotal event of sexual maturation may occur at night while he’s asleep. He wakes up with damp pajamas and sheets, wondering if he’d wet the bed.

A nocturnal emission, or “wet dream,” is not necessarily the culmination of a sexually oriented dream. Explain to your son that this phenomenon happens to all boys during puberty and that it will stop as he gets older. Emphasize that a nocturnal emission is nothing to be ashamed of or embarrassed by. While you’re at it, you might note that masturbation is normal and harmless, for girls as well as boys.

Erections, too, are unpredictable during pubescence. They may pop up for no apparent reason—and seemingly at the most inconvenient times, like when giving a report in front of the class. Tell your teen there’s not much he can do to suppress spontaneous erections (the time-honored technique of concentrating on the most unsexy thought imaginable doesn’t really work), and that with the passage of time they will become less frequent.

Voice Change

Just after the peak of the growth spurt, a boy’s voice box (larynx) enlarges, as do the vocal cords. For a brief period of time, your son’s voice may “crack” occasionally as it deepens. Once the larynx reaches adult size, the cracking will stop. Girls’ voices lower in pitch too, but the change is not nearly as striking.

Breast Development

Breast development in boys? Normal? It is, but try telling that to a young man afflicted with gynecomastia, which literally means growth of the male mammary glands.

Early in puberty, most boys experience soreness or tenderness around their nipples. Three in four, if not more, will actually have some breast growth, the result of a biochemical reaction that converts some of their testosterone to the female sex hormone estrogen. Most of the time the breast enlargement amounts to less than half an inch and is restricted to the nipples, “but in a lean youngster,” says Dr. Norman P. Spack, “gynecomastia can show up quite profoundly.”

As you might imagine, this development can be troubling for a youngster who is in the process of trying to establish his masculinity. “They can be teased mercilessly,” says Dr. Spack, a pediatrician and adolescent-medicine specialist at Children’s Hospital in Boston. If your son suddenly seems self-conscious about changing for gym or refuses to be seen without a shirt, you can reasonably assume that he’s noticed some swelling in one or both breasts. (One particularly telltale sign: wearing a shirt to go swimming.)

Boys are greatly relieved to learn that gynecomastia usually resolves in one to two years. “Thanks for telling me! I thought I was turning into a girl!” is a common reaction. There are rare instances where the excess tissue does not subside after several years or the breasts become unacceptably large. Elective plastic surgery may be performed, strictly for the young patient’s psychological well-being.

Gynecomastia warrants an evaluation by a pediatrician, especially if it arises prior to puberty or late in adolescence, when the cause is more likely to be organic. A number of medical conditions can cause excessive breast growth, including endocrine tumors, the chromosomal disorder Klinefelter syndrome, thyroid disease and herpes zoster, better known as shingles. Breast development may also be a side effect of various drugs, including certain antidepressants and anti-anxiety medications, insulin and corticosteroids. Or a boy may not have true gynecomastia after all, but rather pseudo gynecomastia, which is common in overweight boys. Fatty tissue, not breast tissue, builds up in the chest area, simulating breasts. “Some of these teenagers can have tremendous amounts of tissue, and it can be extremely embarrassing,” says Dr. Spack.

Last Updated
Caring for Your Teenager (Copyright © 2003 American Academy of Pediatrics)
The information contained on this Web site should not be used as a substitute for the medical care and advice of your pediatrician. There may be variations in treatment that your pediatrician may recommend based on individual facts and circumstances.
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