In neonates and infants, the stretched penile length is at least 2cm in 97% of boys.
Micropenis describes a shorter penis than this, that is otherwise of normal form. Penis needs to be stretched out, and suprapubic fat pad pushed in.
Causes are hypogonadotrophic (Kallman’s syndrome, Laurence-Moon-Biedel-Bart, Prader-Willi) or hypogonadism (anorchia or testicular dysgenesis, Trisomy 21, Noonans, Klinefelter). May be part of more complex syndrome.
Differential is intersex, “buried penis” due to suprapubic fat pad (usually obese), chordee.
Neat trick is to modify a 10ml syringe by cutting off needle end and inserting plunger into cut end. Gives you scale and stretches penis!