The causes of the decline in fecundability with age

Abstract Demographers have frequently suggested that women's fecundability reaches a maximum some time in their twenties and thereafter remains at a plateau for some years. Recent work by Bendel and Hua (1978) reinforces the author's previously expressed suspicion that this theory is incorrect. The present note is an effort to synthesize current knowledge about the variation with age of the reproductive parameters underlying fecundability. It is suggested (1) that the major cause of the decline in fecundability across ages 20–40 is a decline in coital rates, and (2) that a smaller part of the decline in fecundability is caused by a rise in risk of unrecognized spontaneous abortion. It is concluded that the “biological capacity to conceive” does not vary much during ages 20–40.