In a study* of 41 nonvegetarian and 34 vegetarian women, carefully matched "with respect to height, weight, body mass index, and menarche, the incidence of menstrual irregularity was 4.9% among nonvegetarians and 26.5% among vegetarians." The probability of a difference this large or larger occurring by chance was nine in a thousand.

_______________

*Petersen, Ann Bryan, et al, "Menstrual differences due to vegetarian and nonvegerarian diets," American Journal of Clinical Nutrition , 53: 879-85, 1991.