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Non-spayed female dogs will go into "heat" or estrus usually twice a year, starting at 6-12 months of age. There are four stages of the canine estrus cycle: proestrus, estrus, metestrus, anestrus.
During proestrus, there will be vaginal bleeding. During estrus, the vulva will be swollen, vaginal discharge is more yellowish, the female will accept males and conception can occur.

www.akc.org/content/dog-care/articles/how-long-are-dogs-in-heat

Jul 30, 2015 ... From about six months old to through the rest of her life, a female dog will experience estrus, or heat, roughly every six months. This is the ...

vcahospitals.com/know-your-pet/estrus-cycles-in-dogs

May 16, 2011 ... When does a female dog have her first estrus cycle? Dogs will have their first estrus cycle when they reach puberty. Estrus is the stage of the ...

www.vetwest.com.au/pet-library/reproduction-in-dogs-season-heat-oestrus-pregnancy-tests

How often do female dogs come into heat? How long does a "heat" cycle or oestrus last? What are the signs of "heat"? How soon after a cycle can she be ...

www.thespruce.com/heat-cycle-for-dogs-3385378

Non-spayed female dogs will normally go into "heat" or estrus twice a year. The age at which their cycles start and the duration of the cycle vary greatly between ...

www.dogbreedinfo.com/breedingheat.htm

Dog Reproduction (The Heat Cycle). Signs of Heat. The first sign of a female ( dam) coming "into season" is often swelling of the vulva. This swelling can occur a ...

dogs.lovetoknow.com/wiki/Female_Dog_Heat_Cycle

Understanding a female's heat cycle can help you prepare for a breeding or ... for brucellosis prior to her next heat cycle, and request that the stud dog owner ...

www.petwave.com/Dogs/Basics/Breeding/Heat.aspx

Female dogs do not menstruate in the way that women do. Instead, dogs have an estrous cycle, more commonly called a heat cycle. Sexually mature female ...

www.askabreeder.com/Dog_Heat_Signs_and_Symptoms

Your dog can get pregnant during her heat cycle (or estrus). ... works very well for the intended purpose and is even listed as 'Useful for female dogs in heat.' ...

www.vetinfo.com/heat-cycle-female-dogs.html

The dog heat cycle can vary dramatically. Dogs go into heat on average twice a year, but some dogs may go into heat three times and many larger breeds only ...