When you enter phrases or key words into the Google Books Ngram Viewer, it displays a graph showing how many times those phrases have occurred in a corpus of books (here English-language books) over the selected years (here 1900 to 2008, last year of data availability). In this Ngram we see that the growth of the volume of books on infertility lags behind the growth of the books on contraception. This graph is one of those illustrating Google evidence of increasing prevalence of impaired fertility = http://biozhena.files.wordpress.com/2012/12/google-evidence-of-increasing-prevalence-of-subfertility.pdf