Main Entry: pro·es·trus
Pronunciation: -'es-tr&s
Variants: or pro·es·trum /-'es-tr&m/ or chiefly British pro–oes·trus or pro–oes·trum /-'Es-/
Function: noun
: a preparatory period immediately preceding estrus andcharacterized by growth of graafian follicles, increased estrogenic activity, and alteration of uterine and vaginal mucosa

proestrus pro·es·trus (prō-ěs'trəs)
n.
The period immediately before estrus in most female mammals, characterized by development of the endometrium and ovarian follicles.