Reproductive hormones are important not only for fertility but also for many aspects of women's health. Substantial variation in hormone levels is observed among individual women and among populations because these hormones are influenced by numerous factors, such as physical activity, weight change and developmental conditions. How do we know what is "normal" and what is outside the norm when it comes to hormone levels? Medicine bases its standards for "normal" on the hormone levels observed in modern industrial populations, but studies in reproductive ecology suggest that these levels are extreme and very different from what our ancestors experienced. Evolutionary medicine offers yet another perspective: We should understand that the levels of hormones that are optimal for fertility are not necessarily optimal for health. Jasienska is the author of The Fragile Wisdom: An Evolutionary View on Women's Biology and Health.