What is “unusually small” for a baby’s penis?

The length of the penis varies considerably between baby boys. Also, for any individual, the penis can appear larger or smaller at different times. Much of the penile tissue can be hidden from view inside the body or under the foreskin. Furthermore, it has been difficult to reach agreement on how to measure the length of the penis. For all these reasons, it’s difficult to say how or when a newborn’s penis can be considered unusually small.

Some clinicians have written that a stretched length of less than 2.5 cm can be considered unusually small in a newborn (Feldman and Smith, Fetal phallic growth and penile standards for newborn male infants. J Pediatr 86: 395, 1975; Taludhar et al, Establishment of a normal range of penile length in preterm infants. J Paediatr Child Health 35: 471, 1998). But this definition is not universally agreed – others say that less than 1.5 cm is a cause for concern.

A chart showing typical human penile growth (stretched length, upper base to tip) is reproduced below:

(Reproduced from Shonfeld & Beebe. 1942, Journal of Urology, 48, 759-777; http://www.caah.chw.edu.au/resources/gpkit/1841_Boys_2-18_Version_3_June_13_08.pdf).

A medical term for unusually small penis is “micropenis”. The prefix “micro-” comes from the Greek word mikros, meaning small; it does not mean to imply something extremely tiny, of the kind that requires a microscope to see.

What causes unusually small penis?

The penis forms during fetal life from the primitive structure called the genital tubercle, under the influence of testosterone produced by the testes. Either insufficient testosterone production or response may therefore contribute to small penis. Insufficient testosterone production may result from complications of testis structure or function, or from inadequate hormonal signalling from the pituitary and hypothalamus. Alternatively, insufficient response to testosterone may occur in milder forms of androgen insensitivity syndrome. Finally, the response of the genital tubercle to testosterone involves pathways of molecular activity that may be perturbed either genetically or due to exposure to certain environmental chemicals.

See also (links not yet active):

Klinefelter syndrome

Androgen insensitivity syndrome (AIS)

Hypopituitarism

Congenital hypogonadotropic hypogonadism

Kallmann syndrome

LH receptor defects

Leydig cell hypoplasia

How common is it?

Medically defined micropenis is thought to affect about 0.6% of adult males.

We have not been able to find figures on the prevalence of micropenis in newborns.

Is it a cause for concern?

The size of the penis at birth is neither a reliable guide to how large it will be in adulthood, nor to how large it will be when erect. The penis will grow considerably by the age of 5, and again in the five or so years after puberty begins. A small penis may not be a cause for alarm. However, a very small penis, as defined above, may compromise sexual function or fertility. A pediatrician can advise on the relative severity of the condition, and whether it may be a sign of complications in other aspects of sexual development.

What is usually done about it?

A small or very small penis may be an indication of insufficient testosterone production or response during fetal life, or part of a broader syndrome, and so might warrant further medical investigation, for example to check whether other signs of incomplete male sex development are present.

Genuine cases of micropenis (that is, not just a small penis) can benefirt from treatment with testosterone (Ishii T et al. Testosterone enanthate therapy is effective and independent of SRD5A2 and AR gene polymorphisms in boys with micropenis". J. Urol. 172: 319, 2004). This is unlikely to provide a complete solution, however.

Enlargement surgery is sometimes considered an option, although in reality this may provide only a cosmetic solution and, like any surgery, carries risks. There are also legal and ethical issues with any surgery of this kind being carried out without the direct consent of the person affected (as discussed under Ambiguous genitalia).

If the most likely cause is found to be hormonal insufficiency or impaired gene function during fetal life, it may be necessary to accept that the critical period during which the size of the penis is determined has passed. It should be borne in mind that the relative size of the penis may become quite different after puberty.

Readers are advised to seek the services of a qualified medical practitioner when considering any diagnosis or treatment options. See Disclaimer.

Further reading

Wikipedia: Human penis size

Wikipedia: Micropenis

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Last updated: 16 July 2015 PK

Edit history: Author P. Koopman 9/12; revised PK 5/13, 10/13