The canine reproductive cycle lasts for six months and usually results in two heat seasons per year. German Shepherd dogs experience monoestrus, or one ovulation per cycle, and, unlike cats, dogs experience spontaneous ovulation which is not triggered by mating.
Canines experience four stages to their cycle: pro-oestrus, which is a preparatory stage before ovulation; oestrus, when the dog ovulates; metoestrus, which corresponds to pregnancy and lactation; and anoestrus, or the quiet phase between heat cycles. Although most canines only experience two heat seasons per year, the German Shepherd can go in and out of cycle with greater frequency, with some cycling into their heat season every four and a half to five months.
The first indication that a female is entering her heat cycle often begins with changes in behavior as she becomes more affectionate and attention seeking. Physical changes also take place as her vulva swells, and she begins to experience a bright red discharge. She will not allow mating to take place during this phase. When the discharge changes color during her mid-cycle, she becomes receptive and this coincides with the time of ovulation. Even though some dogs enter into their heat season with greater frequency, the length of the heat cycle is roughly between three to four weeks in duration.