Fecundity is defined in two ways; in human demography, it is the potential for reproduction of a recorded population as opposed to a sole organism, while in population biology, it is considered similar to fertility, the natural capability to produce offspring, measured by the number of gametes, seed set, or asexual propagules. A lack of fertility is infertility while a lack of fecundity would be called sterility. Human demography considers only human fecundity, at its culturally differing rates, while population biology studies all organisms. The term fecundity in population biology is often used to describe the rate of offspring production after one time step. In this sense, fecundity may include both birth rates and survival of young to that time step. While levels of fecundity vary geographically, it is generally a consistent feature of each culture. Fecundation is another term for fertilization. Superfecundity or retrofecundity refers to an organism's ability to store another organism's sperm and fertilize its own eggs from that store after a period of time, essentially making it appear as though fertilization occurred without sperm. Fecundity is important and well studied in the field of population ecology, though it is studied from a neutral perspective. Fecundity can increase or decrease in a population according to current conditions and certain social factors. For instance, in times of hardship for a population, such as a lack of food or high temperatures, juvenile and eventually adult fecundity has been shown to decrease. Additionally, social trends and societal norms may influence fecundity, though this influence tends to be temporary. Indeed, it is considered impossible to cease reproduction based on social factors, and fecundity tends to rise after a brief decline. Fecundity has also been shown to increase in ungulates with relation to warmer weather. In sexual evolutionary biology, especially in sexual selection, fecundity is contrasted to reproductivity. In obstetrics and gynecology, fecundability is the probability of being pregnant in a single menstrual cycle, and fecundity is the probability of achieving a live birth within a single cycle.
