Between the years of 1996 and 1999, approximately 30,000 nurses volunteered to provide blood and urine samples to the study. Of these women, 18,500 were pre-menopausal, providing samples at specific points in the menstrual cycle. These data allowed researchers to study how hormone levels influence the risk of disease. A second set of samples was collected from 16,500 of the same group of women in 2010-2012, by which time most of them were postmenopausal. Over 25,000 children of women in the Nurses' Health Study II cohort were enrolled in their own follow-up study called the Growing Up Today Study, or GUTS, which has followed the subjects through their early life so as to obtain a second generation of data.