Hair follicles start developing in the third month of pregnancy. By the time a child is born, on average 100,000 hair follicles have developed on its head, equivalent to 1,000 follicles per square centimetre of scalp. Follicle density declines from birth onwards, in line with the growth of a child's scull. As an adult, the total number of follicles declines to some 500 follicles per square centimetre by the time a person reaches the age of 25, to 150-250 between 30 and 50, further declining as we get even older.
Hair follicles grow in random groups over the whole scalp. These groups are known as "follicular units" and consist of 1-5 hairs. On average, each follicular unit has 2-3 hairs.
Lanugo hair is the fine, soft woolly kind of hair that grows all over the human foetus. In most cases, this is replaced shortly before birth by vellus hair, fine “peach fuzz” without pigment. During puberty, the increase in androgenic hormone levels causes the thinner and shorter vellus hair to be replaced with terminal hair in certain parts of the human body. Terminal hair is found not just on the head - beards, nostril and ear hairs, eyelashes, eyebrows, armpits and pubic hair as well the hair on our arms and legs all consist of terminal hair.