Which of the following is NOT one of the four stages of the estrous cycle?

The four stages of the estrous cycle are proestrus, estrus, diestrus, and anestrus. Gestation, however, refers to the period of development of the fetus within the uterus after fertilization has taken place. It is a phase that occurs following successful mating and does not represent a stage of the estrous cycle itself.

Proestrus is the phase preceding estrus where follicles are developing and hormones are changing, estrus is the phase of sexual receptivity or "heat," and diestrus follows, during which the body prepares for potential pregnancy or returns to a state of inactivity if pregnancy does not occur. The estrous cycle is specifically about the reproductive cycle of females when they are not pregnant, whereas gestation is entirely separate and signifies pregnancy. Therefore, gestation is the correct answer as it is not a stage of the estrous cycle.