Which stage defines when pregnancy occurs in an animal?

The stage that defines when pregnancy occurs in an animal is diestrus. During this phase, the animal's body prepares for a potential pregnancy, and if fertilization has taken place, the hormonal changes that support gestation become prominent. Diestrus follows estrus and is characterized by the presence of either a developing embryo or the receptors and hormones needed to sustain a pregnancy.

At this stage, the corpus luteum (the structure that forms in the ovary after ovulation) produces progesterone, which helps maintain the uterine lining and supports the developing fetus. If there has been no fertilization, the diestrus stage will eventually lead to the animal coming back into heat or entering an estrus cycle again after the hormonal changes subside.

In contrast, the other stages mentioned are related to different parts of the reproductive cycle but do not indicate a state of pregnancy. Proestrus is the stage before estrus when the female's body is preparing for mating, metestrus occurs immediately after estrus and deals with the body transitioning away from the mating phase, and anestrus is a pause between reproductive cycles where the body is not preparing for or maintaining a pregnancy.