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Ages & Stages

Physical Development in Boys: What to Expect

Puberty – it's a crazy time and occurs through a long process, beginning with a surge in hormone production, which in turn causes a number of physical changes. Every person's individual timetable for puberty is different. Below is an overview of some physical changes boys can expect during these years.

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

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. Boys tend to look a little chubby and gangly (long arms and legs compared to the trunk) just prior to and at the onset of puberty. They start to experience a growth spurt as they progress further into puberty, with the peak occurring during the later stages of sexual maturation. Body proportions change during this spurt, as there is rapid growth of the trunk, at the legs to some extent too. 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. See Concerns Boys Have About Puberty.

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 parents' choice or religious custom.

"What Are These Bumps On My Penis?"

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


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.

Wet Dreams & 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 child 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. See Nocturnal Enuresis in Teens.

What parents can do to help:

  • 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.
  • Note that masturbation is normal and harmless, for girls as well as boys, as long as it is done privately.

Erections, too, are unpredictable during puberty. 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

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 a firm breast bud of up to 2 inches in diameter under the nipples.  Occasionally, this may be more extensive, resulting in profound "gynecomastia." Overweight boys may have the appearance of pseudo-gynecomastia (lipomastia), due to excess fatty tissue on the chest wall.

As you might imagine, this development can be troubling for a child who is in the process of trying to establish his masculinity. 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
  • An adrenal disorder (congenital adrenal hyperplasia)
  • A chromosomal disorder (Klinefelter syndrome)
  • Liver disease
  • Rare genetic disorders

Breast development may also be a side effect of various drugs, including certain antidepressants, anti-anxiety medications, anti-reflux medications, or due to exposure to external sources of estrogen or estrogen precursors, including ingested soy, and plant estrogen in lotions and/or personal care products, such as lavender or tea tree oil applied to the skin. There may be other possible environmental sources, some of which are under investigation, such as certain plastic containers.

Additional Information:

Last Updated
Section on Endocrinology (Copyright © 2014 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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