Male pattern baldness is also known as androgenetic alopecia. It is the most common cause of hair loss and is associated with a genetic predisposition. The hair loss in androgenetic alopecia occurs gradually and is permanent. Telogen effluvium is reversible hair loss that occurs when the body undergoes stress such as pregnancy, divorce, or extreme dieting. Such hair loss occurs suddenly and it may seem as of clumps of hair fall out at once.