In demography and biology, fecundity is the actual reproductive rate of an organism or population, measured by the number of gametes (eggs), seed set, or asexual propagules. Fecundity is similar to fertility. Demography considers only human fecundity which is often intentionally limited through contraception, while biology studies all organisms. Fecundity is under both genetic and environmental control, and is the major measure of fitness. Fecundation is another term for fertilization. Superfecundity refers to an organism's ability to store another organism's sperm (after copulation) and fertilize its own eggs from that store after a period of time, essentially making it appear as though fertilization occurred without sperm (i.e. parthenogenesis).[citation needed]Fecundity is important and well studied in the field of population ecology. Fecundity can increase or decrease in a population according to current conditions and certain regulating factors. For instance, in times of hardship for a population, such as a lack of food, juvenile and eventually adult fecundity has been shown to decrease (i.e. due to a lack of resources the juvenile individuals are unable to reproduce, eventually the adults will run out of resources and reproduction will cease).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.