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Br April Cashin-Garbutt, BA Hons (Cantab)

Many men worry about the size of their penis; however, not many realise that there is an actual medical condition where the size of the penis is referred to as a “Micropenis”.

But how small actually is a Micropenis? What causes the condition? And how is it treated?

How small is a Micropenis?

As the prefix ‘micro’ suggests, people with a Micropenis have a relatively small penis. But how small exactly is a Micropenis?

The condition refers to any penis which, when stretched, is 2.5 standard deviations below the mean size for the age of the patient. (1)

What does 2.5 standard deviations below the mean work out as?

In adults, the condition refers to any penis smaller than 2.8 inches in length. (2)

In infants, a Micropenis is classed as any penis that is less than 0.75 inches in length. This is considered significantly smaller than a “normal” male newborn’s penis, which is between 1.1 and 1.6 inches in length when stretched gently. (3)

What problems does having a Micropenis cause?

Having a micro-penis can cause several problems, including difficulty urinating and having sexual intercourse.

Fertility can also be affected. Some people with Micropenis have a low sperm count which results in infertility or decreased fertility. (4)

The condition can also have a major impact psychologically. Many men with the condition have very low self-esteem and some even suffer from depression. (2)

What causes Micropenis?

Micropenis is caused by the male baby’s penis failing to elongate after the first trimester of pregnancy. (5)

The cause of this is thought to be a hormonal problem. Specifically, it is thought to be due to insufficient levels of testosterone, a male sex hormone.

The inadequate levels of testosterone may come as a result of inadequate production of testosterone during the second and third trimesters of pregnancy; or as a result of the unborn child not responding to the produced testosterone. (6)

Research carried out on Japanese patients and published in the Journal of Clinical Endocrinology & Metabolism, found that genetic mutations of the SRD5A2 gene can cause Micropenis. (8)

The SRD5A2 gene, or the steroid-5-alpha-reductase, alpha polypeptide 2, codes for the enzyme steroid 5-alpha reductase 2. This enzyme processes the hormone testosterone to dihydrotestosterone (DHT), an androgen which is more potent. (9)

Researchers have also found that there may be a genetic condition which makes boys more susceptible to the development of Micropenis triggered by factors in the environment.

Specifically, they found a polymorphism in the gene for the aryl hydrocarbon receptor repressor (AHRR), called the Pro185Ala polymorphism, which may cause susceptibility to the development of Micropenis in response to dioxins, a type of environmental endocrine disruptor. (7)

Moreover, there is even further research to suggest that newborn male external genital malformations, such as Micropenis, may, in part, be caused by environmental chemicals such as pesticides. (10)

Incidence of Micropenis

Despite not being commonly discussed, the condition is thought to affect 1 in 200 males that are born. (2)

Historical treatment of Micropenis

Historically, Micropenis was treated by gender reassignment of the baby at birth. (6)

This meant that parents were advised to bring their genetically male children up as females. (11)

John Money, the founder of the Gender Identity Clinic at John Hopkins University Medical Centre in Baltimore, was an advocate of gender reassignment. (12, 13)

In 1975, he published a paper outlining the success of a gender reassignment study, in which a twin son had been successfully brought up as a girl. (11, 12)

Money’s study was, however, later discredited by Milton Diamond and later journalist Colapinto, who both showed that the gender re-assignment had not been a success. (12)

Specifically, the genetically male child who had been brought up as a female, decided to change back to her original gender when she found out the truth about her gender re-assignment. Moreover, in 2004, the patient took his own life. (12)

Treatment of Micropenis

Nowadays, according to Mairi MacDonald, the UK Intersex Association (UKIA) views cosmetic surgery of the genitals to reassign an infant’s gender from male to female, solely due to the child having a small penis, as “child abuse”. (5)

Instead, they advocate hormone therapy called testosterone replacement. This can produce a penile length within the “normal range”. (5)

Micropenis can also be treated with surgery called phalloplasty. (2)

Phalloplasty often involves using skin from the patient’s forearm. This skin is wrapped around the native penis and an inflatable penile prosthesis is inserted to provide an erection. (6, 14)

Researchers have found that this procedure produced a penis which was able to be used for sexual intercourse on a regular basis and that remained stable for the long-term. (14)

The procedure is, however, high risk and can lead to complications. (6)



Further Reading

  • All Penis Content
  • Penile Curvature – Peyronie’s Disease, Dupuytren’s Contracture
  • Hypospadias outcomes
  • Hypospadias
  • Hypospadias Research

Last Updated: Jun 28, 2019

Written by

April Cashin-Garbutt

April graduated with a first-class honours degree in Natural Sciences from Pembroke College, University of Cambridge. During her time as Editor-in-Chief, News-Medical (2012-2017), she kickstarted the content production process and helped to grow the website readership to over 60 million visitors per year. Through interviewing global thought leaders in medicine and life sciences, including Nobel laureates, April developed a passion for neuroscience and now works at the Sainsbury Wellcome Centre for Neural Circuits and Behaviour, located within UCL.

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