**2. Normal hair growth**

To fully understand hair loss during childhood, a basic comprehension of normal hair growth is necessary [3, 4]. Generally, at 22 weeks of intrauterine life, the developing fetus has all of its hair follicles formed. At this stage, there are nearly about one million of hair follicles on the head. Hair on the scalp grows about 0.3–0.4 mm/day or about 6 inches/year.

At any given time, a random number of hairs will be in one of three stages of growth and shedding: anagen, catagen, and telogen. Each phase has specific characteristics that determine the length of the hair. All three phases occur simultaneously; one strand of hair may be in the anagen phase, while another is in the telogen phase.

Essentially, there are three basic groups of hair based on hair follicle size [5].

(A) Lanugo hair is long, unpigmented, and very fine, and the very first hair fiber to be produced is by a hair follicle. As an embryo develops, the hair follicles form and begin to produce this type of hair. This first wave of growth is normally shed by the embryo at around 8 months gestation and replaced by terminal or vellus hair ready for birth. However, sometimes babies can be born with this coat of lanugo hair (called "congenital hypertrichosis lanuginosa").

(B) Vellus is short, fine, unpigmented hair. This type of hair is commonly seen on the nose and over the cheeks.

(C) Terminal, it is long, coarse, pigmented, and frequently contains a medulla. During puberty, many hair follicles around the genitals, armpits, beard, and chest in men transform from vellus hair to terminal hair under the direction of hormones. Equally, hormones can cause terminal hairs to revert to vellus hair production as in androgenetic alopecia.
