Regular haircut
Human scalp hair grows on average about one eighth inch per week or one half inch per month. Most clipper guards are numbered in eighths of an inch. The number of the guard denotes the number of weeks of hair growth left on the scalp when a clipper with a certain numbered guard is guided over the head with the guard in contact with the scalp. A #1 guard leaves 1/8 inch (3 mm), one week's growth of hair, on the scalp; a #2 guard leaves 2/8" (6.3 mm), two weeks' hair growth, on the scalp; a #3 guard leaves 3/8" (9 mm), three weeks' hair growth on the scalp; and so on.
Prostaglandin D2
A 2012 research paper indicates a causal link between elevated levels of localized PGD and hair growth inhibition. Applied topically, the researchers found PGD prevents hair growth, and mice that were genetically inclined to produce higher levels of PGD had inhibited hair growth. The researchers also found PGD levels were much higher in balding scalp tissue than nonbalding scalp tissue, through increased levels of prostaglandin D2 synthase. The paper suggested that inhibition of hair growth involved binding of PGD to a receptor called GPR44, and that GPR44 therefore would be a therapeutic target for androgenic alopecia in both men and women with hair loss and thinning. Because PGD's relation to asthma has been known for several years, several drugs that seek to reduce the effect of PGD through blocking the GPR44 are already in clinical trials.