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How does fertility differ from fecundity in population biology?

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Line graph showing world population growth from 1700 to 2000 with a steep rise after 1950, followed by three projection scenarios for 2000 to 2100: high, medium, and low, reaching between 7 and 14 billion. — population Graph of the world's estimated human population from 1700 until 2000, with population projections extending until 2100.

In population biology and demography, it is essential to distinguish between fertility and fecundity, as they represent different aspects of reproduction.12

Fecundity refers to the underlying biological potential for reproduction—the theoretical maximum number of offspring that a species member could produce within a given time period.12
Fertility refers to the actual, realized level of reproduction—the average number of offspring produced by an individual or species member under specific environmental and social conditions.12

The gap between fecundity and realized fertility is often significant, as fertility is constrained by various factors. In humans, these include cultural traditions, socioeconomic conditions, the use of contraception, marriage patterns, and individual choices regarding family size. Biologically, fertility is also influenced by factors such as the age of onset of puberty, the duration of temporary infecundability (such as during breast-feeding), and the onset of permanent sterility.123

It is important to note that these terms can be confusing, as their definitions are sometimes used with opposite meanings in other languages, such as French, or inconsistently across different scientific disciplines.2