Development of the male external genitalia including the penis, testes and scrotum is a step by step process that begins even before birth and continues through the life. It mainly depends upon dihydrotestosterone, which is produced by the testes.
Both genetic and hormonal factors contribute to male genital development. If the fetus is genetically predisposed or if hormonal production from the testes is compromised during gestation (or during puberty) genital development disorders can occur.
Penis development during pregnancy (before birth)
Development of the male external genitalia is dependent upon dihydrotestosterone which is produced by the testes of the male fetus. The foreskin is formed in the twelfth week of development as a thin layer of skin surrounding the penis. During this time, the penis is also developing its corpus cavernosa and spongiosa.
Penis development at birth
In the newborn, the foreskin is attached to the glans.
Penis development during childhood / before preadolescence
Preadolescence is the period of childhood just before the onset of puberty, often designated as between the ages of 10 and 12 in girls and 11 and 13 in boys. During this period, the testes, scrotal sac, and penis have a size and proportion similar to those seen in early childhood.
Penis development at preadolescence
In boys, preadolescence typically occurs at 11 to 13 years of age. As the preadolescence occurs, there is enlargement of the scrotum and testes and a change in the texture of the scrotal skin. Thinning and reddening of the scrotum occurs around 12 years old. Also during this time, the body takes on a more muscular and angular shape because of testosterone. This generally begins around age 12.5 when testosterone causes muscle mass to increase.
Penis development at puberty
Puberty is the appearance of secondary sexual characteristics in males and females. This typically occurs in boys from 13 years of age and onwards. It is initiated by hormone signals from the brain to the gonads – which in men are the testes. In response, the gonads produce a variety of hormones that stimulate the growth, function, or transformation of brain, bones, muscle, skin, and reproductive organs.
In boys, testicular enlargement is the first physical manifestation of puberty. Testicular size continues to increase throughout puberty, reaching maximum adult size about 6 years later. Within months after the growth of the testes begins, rising testosterone levels promote growth of the penis and scrotum.
At this stage and age, the penis is significantly enlarged in length and circumference, with further development of the glans penis. The testes and scrotum continue to enlarge, and there is distinct darkening of the scrotal skin. The penis continues to grow until about 18 years of age, reaching an average stretched adult size of about 13 cm.
Erections and emissions: Males have spontaneous penis erections throughout their lives (even throughout infancy), but during puberty, boys tend to get erections more frequently. Erections can occur with or without any physical or sexual stimulation, and often lead to nocturnal emissions (“wet dreams”), accompanied by a markedly increased libido.