New tests could tell women how many years away from menopause they are, offering them a "roadmap" to their reproductive life.
The University of Michigan studied changes in hormones FSH and inhibin B, which stimulate eggs, in more than 600 women over 14 years.
It found hormones dropped significantly five years before menopause, meaning she was also at her least fertile.
Scientists also tested another 50 women each year for changes in the hormone AMH, which is already used as a predictor of fertility.
The hormone AMH fell to a very low or non-measurable level five years prior to the final menstrual period.
Researcher Maryfran Sowers said the results of this study could help women choosing to have later-in-life babies.
"The information provides a roadmap as to how fast women are progressing through the different elements of their reproductive life," Sowers said Wednesday. "We finally have numbers from enough women evaluated over a long time period to describe the reproductive ageing process."
But IVF Australia chairman Michael Chapman said a reduction in the hormone AMH doesn't necessarily indicate menopause is coming.
"Our experience is, it might be that the ovaries are running short of eggs but it doesn't mean that menopause is due to start in the next one or five years," he said. "Menopause could still be years off."