What is a microphallus?

Updated: Nov 30, 2020
  • Author: Karen S Vogt, MD; Chief Editor: Sasigarn A Bowden, MD  more...
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Answer

Microphallus, or micropenis, is defined as a stretched penile length of less than 2.5 standard deviations (SDs) below the mean for age. Traditionally, the term micropenis refers to a penis that is otherwise normally formed, and the term microphallus has been used when associated hypospadias is present.

The mean stretched penile length in a full-term newborn male is 3.5 cm. Measurements of less than 2-2.5 cm (2.5 SDs below the mean) in a full-term newborn male meet the definition of micropenis and warrant evaluation. [1, 2, 3, 4, 5] Penile growth is essentially linear during mid-to-late gestation. Tuladhar et al (1998) reported the following formula to describe the relationship between penile length and gestational age for infants born at 24-36 weeks gestation: [6]

Penile length in centimeters = -2.27 + 0.16 X (gestational age in weeks)

Although micropenis can be considered a form of ambiguous genitalia, the presence of a normal scrotum and palpable testes indicates a high probability of a normal male karyotype. If the testes are not palpable and/or the penile urethra is absent, the examination is better described as ambiguous, and an evaluation and counseling for disorders of sex development should be performed.

After the first few years of life, the penis grows very little until puberty, when testosterone levels begin to rise. Mean stretched penile lengths and 2.5 SDs below the mean for various age groups can be found in the popular Harriet Lane Handbook (table EC 10.E, 21st ed.). [7]

Occasionally, older boys are brought for evaluation because of concerns of small genitalia. These boys are usually prepubertal and have obesity. Most often, these individuals have normal penis size based on stretched penile length, and the apparent smallness is secondary to the penis being concealed in the suprapubic fat pad ("buried penis"). However, if the penis does measure less than 2.5 SDs below the mean (approximately 4 cm) or other abnormalities are present, such as cryptorchidism or hypospadias, further evaluation is indicated.


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