Micropenis is a medical term for a penis that is normally formed but smaller than the standard size for a male’s age and developmental stage. It is a rare condition affecting approximately 0.6% of males globally. The term refers to the size of the organ, not its structure or function, and is identified by a healthcare provider during a physical examination after birth.
Defining and Diagnosing Micropenis
A diagnosis of micropenis is made by a specialist, such as a pediatric endocrinologist or urologist, through a physical examination. The measurement used is the stretched penile length (SPL). This is obtained by gently stretching the penis to its full length and measuring from the pubic bone to the tip of the glans, providing a consistent measurement to compare to established standards for an infant’s age.
The clinical standard for diagnosis is a measurement that falls more than 2.5 standard deviations below the average for the infant’s gestational age. For a full-term newborn male, the average SPL is about 3.2 cm; a diagnosis of micropenis would be considered if the length is less than 2.4-2.5 cm. For a newborn at 30 weeks gestation, the average length is 2.5 cm, with a micropenis defined as less than 1.5 cm.
Micropenis must be differentiated from conditions that make the penis appear small, such as a “buried penis.” A buried penis is of a normal size but is hidden by an excess pad of fat in the pubic area. Pressing down on the fatty tissue reveals the penis’s true length, and this condition is managed differently as it is not caused by hormonal issues.
Other conditions like hypospadias or ambiguous genitalia may be present alongside a micropenis, but they are distinct diagnoses. To identify the underlying cause, blood tests to check hormone levels and genetic testing may also be performed.
Underlying Hormonal and Genetic Causes
The development of a micropenis is most often linked to hormonal issues during the second and third trimesters of fetal development. Penile growth during this period is heavily dependent on androgens, particularly testosterone. An interruption in the production or action of these hormones can impede normal growth.
The most frequent cause is hypogonadotropic hypogonadism, where the issue originates in the brain’s hypothalamus or pituitary gland. These structures fail to produce sufficient amounts of gonadotropin-releasing hormone (GnRH), luteinizing hormone (LH), and follicle-stimulating hormone (FSH). These hormones signal the testes to produce testosterone, and without adequate signaling, testosterone production is diminished.
A less common cause is primary hypogonadism, where the testes cannot produce enough testosterone despite normal brain signaling. In other rare instances, the body may produce enough testosterone, but penile tissues cannot respond to it, a condition known as partial androgen insensitivity syndrome. Micropenis can also be a feature of genetic syndromes like Prader-Willi or Kallmann syndrome, which are associated with hormonal dysfunctions.
Medical Treatments and Interventions
The most common treatment for micropenis is testosterone hormone therapy. This treatment is most effective during infancy, a period known as “mini-puberty,” when the body is naturally primed for penile growth. The therapy’s goal is to stimulate growth and bring the penile length into a low-normal range for the child’s age.
Testosterone is given as a short course of treatment, often involving a series of intramuscular injections over several weeks or months. A common regimen might involve three injections of 25-50 mg of testosterone spaced four weeks apart. Topical testosterone creams or gels can also be used, and this early intervention can lead to a significant increase in penile length, with an average increase of 75% from the initial size.
This initial treatment in infancy helps prime the penis for further growth during puberty. If the underlying issue is with the pituitary gland’s signaling, other hormones like human chorionic gonadotropin (hCG) may be used to stimulate the testes directly. Surgical options like phalloplasty are far less common, typically reserved for severe cases or for adults who did not respond to hormone therapy.
Long-Term Outlook and Function
The long-term outlook is positive, particularly with successful early hormonal treatment. The penis is expected to grow further during puberty under the influence of naturally increasing hormone levels. While the final adult size may be smaller than average, it is often within a functional range.
Functionally, a micropenis does not inhibit the ability to urinate while standing or to achieve an erection. With successful treatment, most individuals report normal sexual function in adulthood. The ability to have children, however, depends entirely on the underlying cause of the micropenis, not on the size of the penis itself.
If the cause was hypogonadotropic hypogonadism, fertility can often be addressed in adulthood with hormonal treatments to stimulate sperm production. If the cause was primary testicular failure, fertility may be more challenging. The prognosis for both function and fertility is tied to the specific diagnosis and the response to early medical interventions.