What percentage of menstruating, healthy women miss a period but are not pregnant?

Varies: I don't know the exact percentage but it is not that unususal. Many women will miss at least one period a year, especially if they are not on birth control pills. Athletic women, anorexic women, very thin women can also miss periods. Age is not mentioned. Is she approaching menopause? Many different facors come into play, but tbhis is most likely nothing abnormal.