There are four stages to the heat cycle. Anestrus is the R and R stage between heats, lasting an average of five to six months. Proestrus is the onset of heat and discharge. Most females refuse mating at this time. Although flirtatious, they discourage males by growling, baring teeth, snapping, running in the other direction or sitting on the target. During estrus, the female accepts the male and invites coitus with the male. This receptive period of four days to a week is when ovulation takes place. Females will flag (move) their tails to the side or curl them completely over their backs. Males can smell the estrus perfume up to three miles from the source. Diestrus is the is the stage that readies the uterus for pregnancy. If fertilization doesn’t occur, this stage soon reverts to anestrus. If pregnancy exists, diestrus continues until whelping. (Successful Dog Breeding, Walkowicz and Wilcox, pp.48-49)