The reproduction of many organisms follows a cyclical pattern, known as the reproductive cycle, which regulates the conception and birth of offspring. Many animals, including salmon, birds, deer and turtles synchronize their reproduction with the seasons in order to optimize the chance of survival for their young. Humans and other higher primates, on the other hand, remain sexually active all year; human fertility is governed by the menstrual cycle.

Unlike most mammals, which are fertile for their entire lifespans, human females stop going through their menstrual cycles when they reach menopause, usually around the age of 50.

In mammals, fish, amphibians and reptiles, the reproductive cycle is governed by the endocrine sytem. Usually, female mammals are only receptive around the time of ovulation, during a period known as estrus. The reproductive cycle can also be affected by environmental factors. Factors such as water temperature, pH, and current, for example, can all affect the reproductive cycle of fish.