Fertility and Fecundity are closely related terms in reproductive biology. Fertility is the “ability” of a man and a woman to reproduce an offspring. In other words, it is the ability of a woman to get pregnant and the ability of a man to make a woman pregnant. And so, in medical science, if a man and women are not able to conceive a child after more than one year of unprotected sex, they are suspected to suffer from infertility.
Fecundity, on the other hand, is the actual realisation of this ability of a man and women. This means if a man and a woman utilise their fertility powers and reproduce, they are fecund. Fecundity is, thus, calculated in terms of the number of babies a couple has. Fecundity of a population can be calculated in a similar fashion (birth rate).
Note that a man and a woman might be fertile, i.e. capable of reproduction, but would decide not to conceive (via protected sex, birth control pills, etc.) and hence would not be fecund. The other way round is not true for if a couple is fecund, they have to be fertile.
These two terms both deal with reproduction of species (most often mammals), but their difference is not so much physiological as semantic. That is, “fertility” deals with the ability to reproduce, while “fecundity” refers to frequency, willingness, and healthiness of a female to undergo the fertilization numerous times. In bygone days, human fecundity allowed family offspring sizes to go up to and past a dozen; in modern times a family with eight offspring from one mother would be described as “fecund.” As a metaphor in, for example, a business environment, a “fertile” situation is one in which the environment invites new ideas, creativity, fresh thinking – it is a “fertile” environment. Fecundity in business refers to an ongoing creativity, a business atmosphere that invites continued innovation, where new products and ideas are being “born” almost daily. Some companies (Google, for example) thrive on continued innovation, while other companies survive on one single product that was introduced and exploited at the right time.
Fecundity and fertility both refer to the ability to produce offspring.
Fertility is the ability to reproduce whether it is talking about a man, woman, or both.
Fecundity is the potential to reproduce and the how much a single person can reproduce i.e. how many children a single woman can give birth to.
"Fertility rate describes offspring per couple, individual or a population whereas fecundity refers to natural ability to reproduce. Fecundity is affected by genetics, and environmental factors such as economic hardships etc. In hard financial times, fecundity goes down. Fertility depends upon emotions, way of life, food habits and intake of alcohol and other drugs."