Why and how does certain body hair stop growing after a certain length?

I think it is kind of programmed to fall out at a certain length. The hairs go through the different phases of growth. I don’t think hair really “stops” growing per se. It’s more like it grows and eventually falls out, but there is a constant amount of new hairs replacing, and growing, and falling out. Eye lashes only have a short time in the growing stages, under your arm a little longer, hair on your head more time.

It’s not a matter of length, but of time. A follicle goes through a cycle of growth and quiescence. How long the growth period lasts determines how long the hair gets. The length of the cycle is different for various places on the body, with follicles on the head growing for up to 7 years.

At the end of the growth phase, the follicle shuts down and the blood supply that sustained hair growth is cut off. The hair will fall out shortly thereafter, the follicle will go dormant for awhile, then the cycle repeats.

@thorninmud, there must be some that keep growing longer than 7 years, mustn’t there? How else would some young women grow hair that falls to their waists or thighs or (in one case that I saw) ankles?

@Jeruba Longer than 7 years is possible, but at an average growth rate of 6 in/year, 7 years of growth would yield 3½ feet of hair. Growth rate varies some by ethnicity, too. Asians typically outpace Caucasians.