Your ponytail is shrinking...or your part's getting wider...or there's more hair in the shower drain. Take heart -- you're not alone. According to New York City dermatologist Neil Sadick, MD, up to 80 percent of women will have some hair thinning in their lifetime, even if only temporarily. It's normal to shed 50 to 100 hairs a day, says NYC dermatologist Doris Day, MD.