Fertility, Fecundity, Fecundability

Abstract Fertility, fecundity, and fecundability are all terms used to describe aspects of the human capacity to conceive. These terms can be confusing, especially because they are not used in consistent ways. Despite the importance of reproduction to individuals and to society, there is no accurate way to assess this capacity at the individual level until a couple actually attempts pregnancy. The assessment of fertile capacity at the population level is nearly as difficult. Fecundability (the mean probability of conceiving in one cycle) is the best epidemiologic measure of fertile capacity and is used to explore factors that affect that capacity. Meanwhile, birth rates are rapidly falling in most countries, leading to concerns about environmental damage to human reproductive capacity. This chapter addresses possible causes for declining birth rates as well as a range of unanswered questions. One specific concern has been the apparent decline in sperm counts over time, which may be related to environmental exposures or to other factors.