The stage when animal comes in heat

The stage when animal comes in heat is called Proestrus.
Proestrus is the preparatory stage for an animal coming into heat, metestrus is a brief period characterized by the decline of corpus luteum functions in the absence of conception.
Proestrus is an anabolic phase in which there is active growth of the genital tract, producing a thickening and venous congestion in the uterine mucosa and at the external orifice of the uterus.