Hair Growth Cycle

• Hair grows one-half inch per month on average, according to Web MD, and each strand of hair has a growth phase of two to six years. A healthy growth cycle consists of four phases, growth, transition, resting and returning growth, says Women to Women. Once a hair completes all four phases, it falls out and the follicle begins a new cycle. Ideally, hairs are replenished.
  
  

Cycle Interruptions

• The primary cause for sudden hair loss or thinning is an interruption in the four-phase hair growth cycle. Most commonly, according to WebMD, a genetic predisposition causes a group of hormones called androgens to disrupt the process. Some hairs prematurely fall out.
  
  

Female Pattern Hair Loss

• Androgenic alopecia, or female pattern hair loss, can also be caused by other enzymes, hormone receptors and blockers. Although WebMD reports that there is evidence linking these elements to female pattern hair loss, more research is needed.
  
  

Other Medical Causes

• Medical conditions also cause female pattern hair loss. Polycystic ovary syndrome, autoimmune disorders, alopecia areata, thyroid disorders and anemia are examples. A condition called telogen effluvium can cause hair loss after childbirth, crash dieting, surgery or trauma, says WebMD.