In demography, fertility indicates the product or output of reproduction, rather than the ability to have children. The physiological ability to have children—that is manifest roughly in the period between menarche and menopause in women—is termed fecundity. Demographers define a third, further aspect of reproduction—fecundability—which is the probability of becoming pregnant, or the likelihood of exposure to the possibility, that depends on the pattern of sexual and pregnancy preventive behaviours.
１月経周期あたりの受胎数。つまり，妊孕力のある受胎待ち時間（fecund waiting time for conception)の逆数である。
Fekundabilität ist die Wahrscheinlichkeit pro Monat auf natürlichem Wege schwanger zu werden.