Puberty and Hair
A: None. Humans are born with all the hair follicles they will ever have: roughly 5 million total across the body, with about 100,000 on the scalp. All hair follicles are fully formed in utero by around 22 weeks of fetal development. We have the same number of hair follicles as adults as we did when we were born. The only difference is the type of hair produced by these follicles.
Vellus vs. Terminal Hair
If you look closely at a baby’s skin (or your own skin in areas that seem hairless), you will see tiny, pale, fuzzy hairs. These are called vellus hairs, which help with body temperature regulation and evaporation of sweat.The hair found on the scalp, eyebrows, and eyelashes that is darker, longer, and thicker is known as terminal hair. Pre-puberty individuals typically have terminal hairs only in these specific areas, with vellus hair covering the rest of the body.
The Puberty Shift
As we enter puberty, the body produces sex steroids; specifically androgens like testosterone (present in both boys and girls, though higher in boys). These hormones trigger changes in the follicles in specific androgen-sensitive zones: the pubic region and armpits in both sexes, as well as the face, chest, abdomen, and back primarily in boys.Under the influence of androgens, the vellus hairs in these areas undergo a process called terminalization: the follicles enlarge and suddenly start producing thicker, darker, and often curlier terminal hair.
Hair Changes Later in Life
Later in life, hormonal shifts continue to alter follicle behavior. As men age, dihydrotestosterone (DHT) can cause scalp follicles to miniaturize, leading to male pattern baldness (androgenic alopecia). In women, shifts in estrogen and androgen balances - especially during menopause - can trigger thinning scalp hair or the development of terminal facial hair.So, puberty doesn't cause the creation of new hair follicles. It merely signals existing follicles to change the type of hair they produce.
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See also:
How to analyze hair
Hair and hormonal changes
How does a girl get rid of small hairs under her nose