1. Genetics:
* Heredity plays a significant role: Your genes largely determine hair density, thickness, and growth rate in all areas, including the underarms and legs. If your parents or grandparents have sparse hair in these areas, you're more likely to have the same.
* Hair follicle density: The number of hair follicles per square inch varies from person to person. Some people are simply born with fewer hair follicles in these areas.
2. Hormones:
* Androgens: These are male hormones, like testosterone, that influence hair growth. While women have lower levels than men, androgens still contribute to hair growth. Fluctuations in androgen levels (due to age, puberty, pregnancy, menopause, or hormonal disorders) can affect hair growth patterns.
* Estrogen: This female hormone can counteract the effects of androgens. High estrogen levels can lead to thinner or slower hair growth.
* Aging: As you age, hormonal changes occur, which can lead to a decrease in hair growth rate and density.
3. Hair Removal Methods:
* Frequent shaving: Shaving doesn't directly reduce hair growth, but over time, it can make the hair feel finer as the blunt cut end tapers off.
* Waxing and plucking: Repeatedly removing hair from the root (as with waxing or plucking) can, over a long period (years), potentially damage the hair follicle and reduce hair growth or lead to thinner, weaker hair. This is because the follicle can become scarred or stop producing hair altogether.
* Laser hair removal and electrolysis: These methods are designed to permanently reduce or eliminate hair growth by targeting the hair follicle.
* Hair removal creams (depilatories): These dissolve the hair shaft at the surface of the skin and do not affect hair growth.
4. Age:
* Puberty: Hair growth increases during puberty due to hormonal changes.
* Adulthood: Hair growth usually stabilizes in adulthood.
* Aging: Hair growth may slow down and become thinner as you age due to hormonal changes and decreased hair follicle activity.
5. Medical Conditions and Medications:
* Medical conditions: Certain medical conditions (such as thyroid disorders, anemia, autoimmune diseases, and polycystic ovary syndrome [PCOS]) can affect hair growth patterns, sometimes leading to hair loss or thinning.
* Medications: Some medications (such as chemotherapy drugs, birth control pills, and certain antidepressants) can also affect hair growth.
* Nutrient deficiencies: Severe deficiencies in certain nutrients (like iron, biotin, and zinc) can affect hair growth, although this is more likely to cause overall hair thinning than just in specific areas.
6. Friction and Clothing:
* Constant friction from clothing (especially tight clothing) can sometimes cause hair to break or fall out more easily, although this is usually not a significant factor in overall hair growth.
Important Considerations:
* Normal variation: It's important to remember that there's a wide range of normal when it comes to hair growth. Some people naturally have less hair in these areas.
* Consult a doctor: If you experience a sudden or significant change in hair growth, or if you have concerns about hair loss, it's best to consult a doctor or dermatologist. They can help determine the underlying cause and recommend appropriate treatment.
* Hair texture: Finer hair might be harder to see than coarser hair.
In summary, a combination of genetics, hormones, age, hair removal methods, and potentially underlying medical conditions can influence hair growth in the underarms and legs.