Fertility is defined as the natural capacity to conceive a child. However, fertility does not come easily to everyone; about 11 percent of couples will face infertility —the inability to conceive naturally after one year of unprotected sexual intercourse.

In demographic contexts, fertility refers to the actual production of offspring, rather than the physical capability to produce which is termed fecundity. While fertility can be measured, fecundity cannot be. Demographers measure the fertility rate in a variety of ways, which can be broadly broken into "period" measures and " cohort " measures.