Fertility and fecundity are two closely related concepts.
One means the physiological state of being able to have offspring, the other means the state of actually producing offspring. In a population of 26 year old human females, 95% might be physiologically capable of giving live birth, while perhaps only 10% might actually have a baby that year. So fertility and fecundity are different. The problem is, which is which?
Demographers refer to the ability of have babies as fecundity, and the rate at which women actually have kids as fertility. Biologist do the opposite, saying that fertility is the capacity and fecundity is the realization.
This is just a quaint fact in academic linguistics until one tries to write papers and give presentations that will sit well with both demographers and biologists. Biologists think it is totally unreasonable to adopt the linguistic oddities of the social scientists, and demographers are not happy about people reversing the meaning of these terms. I usually go with the demographer's lingo. They are the ones who employ me.