What Is a Micropenis?

Micropenis is a term used to describe a penis that is 2.5 standard deviations smaller than the mean (average) for the age and race of your child. This translates to a stretched penis length of less than 1.9 centimeters (3/4 inch) at birth; among adults, a micropenis is defined as a flaccid penis length of 7 centimeters (2-3/4 inches) and an erect penis length of 12.5 centimeters (5 inches). During fetal development, deviations may occur as a result of a genetic aberration or be triggered by hormonal abnormalities.

Statistically, micropenis occurs in 0.6 percent of the population, making it a rare characteristic. The term is most commonly used when all other structures of the genitals, including the scrotum, testicles, and perineum, are "normal."

A micropenis develops during gestation and will often the only physiological abnormality noted in the pregnancy.

One of the possible causes of this is the low production of human chorionic gonadotropin (hCG) during the early part of the pregnancy. This is a hormone that stimulates the developing testes to produce testosterone.

After 14 weeks, penile growth falls under the influence of another hormone, known as luteinizing hormone (LH), which also stimulates testosterone in the so-called Leydig cells of the testicles. If the production of either or both of these fetal hormones is impeded, the child's penis length may be affected.

Genetics may also play a role. While there is no single gene that causes a micropenis, the condition is commonly linked to such chromosomal disorders as androgen insensitivity syndrome (AIS), Klinefelter syndrome, Turner syndrome, and Down syndrome.

There is also evidence that estrogen-based fertility drugs like diethylstilbestrol (DES) may cause smaller-than-normal penis size if taken during early pregnancy. While environmental pollutants are a less common cause, some research suggests that exposure to chlorinated pesticides during pregnancy may cause micropenis and other genital abnormalities in male babies.

In newborns, it is vital that a doctor takes a proper measurement of the baby's penis when diagnosing micropenis. As opposed to a flaccid penis length (FPL), in which the penis is laid parallel to a ruler, a stretched penis length (SPL) should be used as it more closely correlates to the erect penis length in boys and men.

For this, the doctor needs to hold a rigid ruler held firmly against the pubis at a right angle. The penis is then held at the sides with calipers just below the glans (head) and stretched to its maximum length without pain. Newer syringe-like tools that can be placed over the penis and suction the organ to its fully stretched length are also available.

Correctly identifying a micropenis in babies is crucial as it offers the opportunity for a potentially effective treatment. The doctor should also explore conditions commonly associated with a micropenis, including problems with the pituitary gland or hypothalamus.

Defining Micropenis

While an SPL of less than 1.9 centimeters is diagnostic of micropenis in full-term newborn, for older boys and men, a micropenis is characterized by penis length 2.5 standard deviations (SD) smaller than the mean average for the age.

As a guideline, the Harriet Lane Handbook from John Hopkins University defines micropenis as follows:

• 6 to 12 months: less than 2.3 centimeters (0.9 inches)
• 1 to 2 years: less than 2.6 centimeters (1.02 inches)
• 2 to 3 years: less than 2.9 centimeters (1.14 inches)
• 3 to 4 years: less than 3.3 centimeters (1.3 inches)
• 4 to 5 years: less than 3.5 centimeters (1.38 inches)
• 5 to 6 years: less than 3.8 centimeters (1.5 inches)
• 6 to 7 years: less than 3.9 centimeters (1.54 inches)
• 7 to 8 years: less than 3.7 centimeters (1.46 inches)
• 8 to 9 years: less than 3.8 centimeters (1.5 inches)
• 9 to 10 years: less than 3.8 centimeters (1.5 inches)
• 10 to 11 years: less than 3.7 centimeters (1.46 inches)
• Adult: less than 9.3 centimeters (3.66 inches)

The variations in mean penis size from age 7 is due to the differences in development as boys approach puberty. By puberty, it becomes far more difficult to define micropenis by centimeters alone; algorithmic calculations are needed until puberty is complete.

Differentiations in Boys

While the clinical definition of a micropenis seems to offer a definitive roadmap for diagnosis, that's not always the case. This is especially true in boys over the age of 8.

In fact, most prepubescent boys brought in by their parents because of an underdeveloped penis rarely have a micropenis. In most cases, the boy is either experiencing delayed puberty, obesity (obscuring the penis length with excessive pubic fat), or he simply has a larger frame compared to an otherwise normal penis.

In cases like these, the term "inconspicuous penis" may be more appropriately applied. It may be secondary to congenital conditions such as penoscrotal webbing (in which the scrotum extends up the underside of the penis, creating an indistinct junction between the two) and megaprepuce (in which the foreskin cannot retract and balloons abnormally).

The treatment of micropenis will vary between children and adults. Given that the genitals of babies and toddlers are still developing, testosterone treatment can support penis growth, often significantly. Surgical options, while limited, may be explored in boys and men whose penises have reached their maximum growth.

Depending on the treatment plan, the medical team may include a pediatrician, urologist, endocrinologist, geneticist, or psychologist.

Testosterone Therapy

A micropenis can be treated in infants and children with three monthly intramuscular (IM) injections of testosterone. Research has shown that one or two courses of three testosterone shots (25 to 50 milligrams) given in four-week intervals could increase the child's penis size to the reference range for his age.

For a male baby with a micropenis, circumcision should be delayed until the testosterone therapy is completed. Generally speaking, the therapy is most effective in children under 3 but may benefit boys up to the age of 8.

Gender Reassignment

In the past, young children with a micropenis often underwent gender reassignment surgery, more in response to a general cultural discomfort with small penis size rather a real medical need. Today, that practice has largely subsided, with most experts questioning its wisdom given the potential benefit of testosterone therapy, the need for lifelong male-to-female hormone therapy in later life, and the lack of individual consent.

If pursued, gender reassignment would generally be considered at a later age when the boy has the ability to make an informed choice for himself and has undergone extensive psychological evaluation.

Penis Enlargement Surgery

Some men with a micropenis will opt to undergo penis enlargement surgery (phalloplasty) with varying degrees of success. One such operation, called a suspensory ligament release, involves the detachment of the ligament that supports the penis during an erection.

Doing so allows the penis to lie in an obtuse rather than acute angle, creating the perception of greater length. Potential risks include nerve damage, loss of penial sensation, erectile dysfunction, and the retraction of the penis if scar tissue develops at the incision site. 

Other forms of phalloplasty, such as flap surgery (grafting skin from another of the body), are less commonly pursued because they carry a considerable risk of complications and may interfere with sexual function.

Other techniques, such as silicone implants (prostheses), synthetic dermal fillers, and subcutaneous fat injections, are more likely to increase the girth rather than the length of the penis. Even if length gains are achieved, it would only affect the flaccid length, not the erect length, which would remain the same.

There are also commercially marketed penis pumps and stretchers that have not demonstrated consistent results in delivering gains in penis length. If achieved, they tend to be modest at best. These devices are intended for males with erectile dysfunction—an entirely separate concern.

From a practical standpoint, micropenis may complicate urination by making it difficult to direct the stream. Many men will simply compensate for this by sitting on the toilet when urinating.

On a more significant note, penis sizes of less than two inches are associated with a lower likelihood of conception. Moreover, some men with a micropenis will have a low sperm count as the result of an underlying pituitary disorder. In such cases, assisted reproductive techniques are available to significantly improve one's chances of conceiving.

A Note About Emotional Development and Relationships

While there are treatments available that may enhance a boy's penis size, the reality is that some boys (and men) will have a smaller-than-normal penis. While some people assume that this will cause inherent psychological harm to a boy, those beliefs are more reflective of our cultural attitudes about penis size rather than the boy's individual experience.

In fact, long-term research has shown that men with micropenises are no different in how they view their masculine self-image compared to men of average or above-average penis size. Moreover, the condition has not been shown to interfere with a man's libido, sexual function, sexual satisfaction, ability to achieve an erection, or the ability to forge mutually gratifying sexual relationships.