Secular changes in age at menarche in a sample of US women

In an epidemiological survey of the reproductive history of a large group of United States women who had given birth to at least one child, there was a secular trend in age at menarche between those born around 1920 and those born around 1940-45. The trend was linear and the increment 3.2 months (+/- 0.36) per decade. The women were White, well nourished, middle-class and resided in all parts of the United States.