Less than 2% of placental mammals menstruate: the elephant shrew, certain bat species, the spiny mouse, and primates. Research shows that throughout human history, women had more offspring, limiting their lifetime menstrual cycles to about 40. But human evolutionary trajectory is changing: in high-income nations in particular, women are having fewer children and can experience up to 400 periods. This makes menstruation and heavy menstrual bleeding more relevant to medicine and humanity than ever.
