Bitches can have their first 'season'/'heat cycle' at 6 months of age, and sometimes as late as two years, although puberty generally begins at around 12 months of age. In large breeds, it can be around 18 months - 2 years. Generally, a bitch will have a cycle every 6 - 12 months, yet sometimes when they get older this can be less frequent.

The signs are

A bloody vaginal discharge is associated with proestrus. Initially, the vulvar lips swell and become soft and pliable.
Restlessness
Frequent urination
Ability of the female to attract males from long distances away
Receptivity to the male

The bitch's cycle is divided into four periods.

Proestrus: The bitch will attract the attention of males, but she will have little interest in them. She will have a bloody vaginal discharge and her vulva will be swollen. Proestrus lasts on average 10 days.

During the first week of female dog in heat you will notice a change in her mood. Some female dogs become too lenient and affectionate towards their owners while some female dogs in heat prefer to remain in melancholy and not be disturbed by any one.

A bitch's behaviour can also change, she may develop a large appetite and or have less interest in food, and sometimes an obedience and well mannered bitch can become grumpy.

Estrus: Simply put this is the bitches 'fertile' period when she will have more of an interest in males and lasts between 5 - 9 days. During Estrus, the vaginal discharge becomes clear and changes into mucus, which indicates a bitch is ready for mating.

Diestrus: Diestrus lasts from 60 to 90 days in a dog that doesn't become pregnant or approximately 63 days in a dog that does become pregnant. Phantom pregnancy, a condition in which the bitch shows symptoms of being pregnant although she has not conceived, is occasionally seen during Diestrus.

Anestrus: Anestrus lasts an average of 4 to 5 months. The sex hormones are low, and there is no sexual activity.

Pyometra is an infection of the uterus which usually occurs 2 to 6 weeks after the Estrus cycle. During Estrus, the cervix is open to allow conception, and some of the body's natural defenses are relaxed allowing infection to enter the uterus from the the vagina. The cervix then often closes down again allowing the the uterus to fill with infectious material or "pus". Bitches exhibit a variety of symptoms, the most common being lethargy, loss of appetite and increased water intake and urination. If the cervix is open, there may be a heavy purulent (pus) vaginal discharge. (more information)

There are two kinds of Pyometra.

Open, being easier to detect because there is a purulent discharge, lethargy, fever, anorexia, and sometimes vomiting.

Closed, This more sinister of the two, and deadly. There is no discharge but the other signs and symptoms are present. The infection remains within the body and diagnosis is difficult because nothing shows externally. Blood work, radiology, ultrasound and physical examination are used to rule out other causes before a definite diagnosis of Closed Cervix Pyometra can be made. With ultrasound, sometimes fluid can actually be seen in the horns of the uterus and white cell counts may be high, leading to the diagnosis.

Both kinds of Pyometra are life-threatening and a total ovariohysterectomy is recommended by most vets. Even if treated medically with prostaglandins, it will almost always recur on the next season or later. The uterus can increase in size dramatically, sometimes exceeding that of a pregnancy, and many bitches that have been bred from are believed to be in whelp, whereas in fact they have a Pyometra.

The wall of the uterus can also become very thin and susceptible to rupture. Left untreated, Pyometra is a fatal condition.

For a valuable brood bitch or one at the height of a show career there is the option of hormone treatment which along with strong antibiotics, will expell the pus from the uterus. Unfortunately the treatment can be dangerous, ast prostaglandins cause the uterus to go into labour to push out the infected pus. There is risk the uterus could rupture, or that the infection could be pushed out through the fallopian tubes into the body cavity causing perionditis.

Treatment with prostaglandins has proved to be successful in a majority of cases of open cervix Pyometra, but unsuccessful in a majority of closed cervix cases. If the uterus ruptures, and there is no alternative but to have the bitch spayed, together with a course of antibiotics.