: a period immediately preceding estrus characterized by preparatory physiological changes.

What happens during Proestrus?

During proestrus the CL regresses (progesterone declines) and a preovulatory follicle undergoes its final growth phase (estradiol increases). Ovulation usually occurs during estrus (cows ovulate during metestrus). Proestrus and estrus comprise the follicular phase.

What is Proestrus and estrus?

The first part of the cycle is proestrus which is characterized by marked changes in the reproductive system but particularly uterine growth and endometrial proliferation. This is followed by the climax of the process, estrus, at the end of which ovulation usually occurs.