The Fertility Clock provides a simple graphic that can be used to estimate the monthly chance of having a baby.
Age-related decline in natural fertility is due to both decreases in egg count and egg quality; egg quality refers to an egg’s ability to create a chromosomally normal embryo.
As seen in the Fertility Clock, monthly birth rates remain relatively stable through age 35 despite a 90% drop in overall egg counts. Thereafter, birth rates decline more rapidly probably as a result of waning egg quality.
The data used to create the Fertility Clock was taken from landmark works in the fields of Population Research and Developmental Biology.
Using data from historical populations (1) and the most comprehensive study of ovarian reserve to date (2), estimates for monthly birth rates and egg counts were plotted in an age-dependent manner, from birth to menopause.
The rates depicted on the Fertility Clock estimate the experience for most women. As with all biological systems exceptions do exist.
(1) Wallace WH, Kelsey TW. Human ovarian reserve from Conception to the Menopause. PLoS One. 2010 Jan 27;5(1):e8772.
(2) Bongaarts, J. and Potter, R.G. (1983). Fertility, Biology, and Behavior: An analysis of the proximate determinants. New York, NY: Academic Press, Inc.