Unlike scalp hair, armpit hair reaches a limited length before stopping its growth. This difference is governed by specific biological processes that dictate hair growth patterns across the human body.
Understanding Hair Growth Cycles
Hair on the human body, including that found in the armpits, follows a cyclical pattern of growth, shedding, and rest. This cycle consists of three main phases: anagen, catagen, and telogen. Each hair follicle operates independently, ensuring that not all hairs are in the same phase at once, which prevents sudden, widespread hair loss.
The anagen phase is the active growth period where hair cells rapidly divide, causing the hair shaft to lengthen. For scalp hair, this phase can last for several years, typically ranging from two to eight years. At any given time, a large majority of the hair on your head, about 85% to 90%, is in this active growth stage.
Following the anagen phase is the catagen phase, a short transitional period that signals the end of active growth. During this stage, which lasts about 10 days to three weeks, the hair follicle shrinks, and hair growth ceases. This phase involves a small percentage of hairs, typically around 1% to 3%.
The final stage is the telogen phase, a resting period where the hair remains in the follicle but no longer grows. This phase usually lasts for approximately two to three months. After this resting period, the old hair is shed, making way for new hair to begin its anagen phase, restarting the cycle.
How Armpit Hair Growth Differs
The primary reason armpit hair does not grow as long as scalp hair lies in the varying duration of its anagen, or active growth, phase. This shorter growth period directly limits the maximum length armpit hair can achieve.
For armpit hair, the anagen phase typically lasts only about 30 to 45 days, or roughly one to one and a half months. This contrasts sharply with the multi-year anagen phase observed in scalp hair. Even though the actual rate at which hair grows per day is similar across different body parts, the restricted timeframe for active growth means armpit hair simply cannot grow indefinitely.
Due to this abbreviated growth phase, armpit hair usually reaches a maximum length of only a few centimeters, or up to about two to three inches. After reaching this length, it enters the transitional and resting phases.
Influences on Armpit Hair Growth
The specific, shorter anagen phase observed in armpit hair is largely influenced by hormonal signals within the body. Androgens, a group of sex hormones that includes testosterone, play a significant role in the development and regulation of hair follicle cycles in these areas. These hormones become more active during puberty, triggering the growth of thicker, darker hair in regions like the armpits.
These androgen hormones interact directly with hair follicle cells, influencing their growth patterns and the duration of the anagen phase. Beyond hormonal influences, an individual’s genetic makeup also plays a considerable part in determining the characteristics of armpit hair.
Genetic predispositions contribute to individual variations in armpit hair, including its maximum length, density, and even its presence. Some people may naturally have less armpit hair or no armpit hair at all due to their inherited genetic traits. Therefore, both the body’s hormonal environment and inherited genes work together to control how armpit hair grows and its ultimate length.