We are born with approximately 100,000 hair follicles and do not develop new ones after birth. Hair follicles change in size over time as they pass through various phases of the hair cycle.
Each hair follicle completes 10 to 20 cycles in a lifetime. There are three classic stages to the hair cycle: anagen, catagen, and telogen. Anagen, or growth phase, lasts 3 to 10 years. Catagen, the trasitional, phase is only a matter of 2 to 3 weeks, while telogen, or resting phase, lasts 3 to 4 months. The majority of hairs remain in growth phase, with an average of 13% and 1% of scalp hairs in telogen and catagen phases, respectively. Hair follicle cycles throughout the scalp are not synchronous.
Dr. Carlos Wesley.