A micropenis is an abnormally small penis. In medical practice, the dimension of the penis that is measured is the length. This measurement is taken along the upper surface of the shaft of the penis to the tip, using a measuring tape or, preferably, a ruler. The ruler is pressed firmly into the soft tissue over the pubic bone (the symphysis pubis) since, in obese boys and men, a seemingly small penis may be partly engulfed by the fat pad at its base and actually be quite normal in length.
Normal standards are available for penile length.A micropenis is more than 2.5 standard deviations below the average size for age. In a newborn, a stretched penile length less than ¾ inch (1.9 cm) is micropenis.
Micropenis is a hormonal problem that takes place sometime after 14 weeks gestation when the penis has already formed. Hormone levels need to be checked. A boy's chromosomes should also be checked to see if there is an underlying genetic syndrome. An MRI may be needed to look at the hormone secreting glands. If the penis grows when a 3 month-trial of testosterone is given, the outlook is good for normal adult penis size and function.