noun
or pro·estrum also pro·oestrus or pro·oestrum “+
( plural proestruses or proestrums )
Etymology: New Latin, from pro- (I) + estrus, estrum, oestrus, oestrum
: a preparatory period immediately preceding estrus and characterized by growth of graafian follicles, increased estrogenic activity, and alteration of uterine and vaginal mucosa