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One of the most challenging things about hair loss is figuring out why it’s happening. The list of causes ranges from genetics to medication to lifestyle. While it can be hard to pinpoint the cause right away, knowing the possibilities can help you figure it out.

Heredity

Most of us can blame Mom and Dad for thinning locks, says Sonia Badreshia-Bansal, clinical instructor in dermatology at University of California, San Francisco, and a dermatologist who specializes in hair loss in women.

“Heredity is the most common cause of hair loss,” she says. “The gene can be inherited from either the mother's or father's side of the family, though you’re more likely to be affected if both of your parents had hair loss.”

Hereditary hair loss affects about 30 million women in the United States, the American Academy of Dermatology says. Women with this trait tend to develop thinning at the hairline, behind the bangs, or they might notice more scalp showing or a widening part, Badreshia-Bansal says. The condition develops slowly and may start as early as your 20s.  

How to know for sure? A scalp biopsy can show if the hair follicles have been replaced with smaller follicles. That's a surefire sign of hereditary hair loss, she says. Applying minoxidil (Rogaine) to the scalp twice a day can stop further thinning, she says.

'Excessive Shedding'

Telogen effluvium is a common type of hair loss translates to excessive shedding. (It’s normal to shed between 50 and 100 hairs a day.)

This type of hair loss can happen after your body goes through stress, says Amy McMichael, MD. She's the chair of the dermatology department at Wake Forest School of Medicine in Winston Salem, N.C.

Possible causes include:

  • surgery
  • general anesthesia
  • a change in medication
  • childbirth
  • a high fever
  • flu
  • severe anemia
  • extreme psychological stress

Women with telogen effluvium typically notice hair loss between 6 weeks to 3 months after the stressful event. At its worst, handfuls of hair may come out. 

Diet can play a role, too. Shortfalls in protein and iron can bring on telogen effluvium. So can extreme weight loss, says Paradi Mirmirani, MD, a dermatologist with Permanente Medical Group in Vallejo, Calif.

There are no tests for telogen effluvium, but your dermatologist may ask you about recent life events and look at the root of hairs you’ve shed. Club-shaped bulbs are a tell-tale sign, says Mirmirani, who's also a member of the North American Hair Research Society. The bulbs mean your hair has gone through a complete growth cycle, which may have sped up due to stress. 

What can you do?

“In some cases, such as pregnancy or major surgery, reassurance and time is the best remedy,” she says. “If medication is the culprit, talk to your doctor about lowering your dosage or switching drugs. If it's stress-related, reduce anxiety.”

And if your diet isn't great, take steps to improve it.  

Hair can start to regrow in about 6 months, if the cause of the effluvium is resolved.