Bitches normally come into season at six-month intervals, each season lasting about 23 days.
It is not known what brings bitches into season, but light and temperature are believed to be important factors.
There are occasions when considerable numbers of bitches of different breeds appear to come into season earlier or later than expected.
There are some bitches that only come into season once every year, and others that come into season every four months.
Such bitches need careful watching if a breeding programme is being planned.
The first stage of the cycle is that the bitch begins to secrete the hormone oestrogen.
As the oestrogen level increases, it causes the bitch to come into season in the next few days.
As this happens, the vulva enlarges and it is at this point that the bitch becomes attractive to dogs.
This attraction is enhanced by the bitch urinating far more frequently.
In the early part of the season, the urine contains a scent which presents a clear signal to males.
As a bitch comes into season she generally becomes more excitable, so care needs to be taken if you have several bitches together, apart from the obvious need to separate the bitch from any males for the duration of the season.
Apart from causing the enlargement of the uterus, the rise in the oestrogen level causes an increase in the blood supply to the uterus and vagina.
The small blood vessels within the vagina then rupture, which leads to the characteristic blood-stained discharge which indicates that the bitch is in season.
The first day that this discharge is present is generally taken to be the first day of the season.
It is important to watch carefully for this, as some bitches are particularly clean and can conceal it for several days.
A bitch may occasionally have a dry, or clear, season in which there is little or no evident discharge.
This does not indicate any problem and such bitches will produce litters in the normal way, provided they are mated on the correct day.
By about the tenth day, the oestrogen level begins to fall and another hormone, progesterone, begins to increase.
This causes the vulva to soften and open, so making the bitch physically receptive to the dog.
At the same time, actual ovulation begins.
The objective is to mate the bitch at the time of maximum ovulation, or slightly before.
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