Understanding the hair growth cycle is fundamental to understanding hair loss and what happens during a hair transplant. Hair does not grow continuously — it cycles through three distinct phases, and disruption of this cycle underlies many forms of hair loss.
The anagen phase is the active growth phase. Hair grows approximately 1.5 centimetres per month during anagen, and this phase typically lasts two to six years. At any given time, approximately 85 to 90% of scalp hairs are in anagen. The duration of anagen determines the maximum length to which hair can grow — individuals with very long hair have a genetically longer anagen phase.
The catagen phase is a brief transitional phase lasting two to three weeks. During catagen, the follicle shrinks and detaches from its blood supply. The hair shaft stops growing and begins to be pushed upward.
The telogen phase is the resting phase, lasting approximately three months. The hair shaft is retained at the surface — this is the hair that falls when combed or washed — before being shed and replaced by the new anagen hair growing from the same follicle. Approximately 50 to 100 hairs are shed daily in normal telogen shedding.
In androgenetic alopecia, DHT progressively shortens the anagen phase in susceptible follicles, while the telogen phase remains the same. Each successive cycle produces a shorter, thinner hair until the anagen phase becomes so brief that the produced hair is microscopic. This is the miniaturisation process.
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