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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.

en.wikipedia.org/wiki/Menstruation_(mammal)

Menstruation in mammals is the shedding of the uterine lining (endometrium). It occurs on a regular basis in sexually reproductive-age females of certain ... Female domestic animals used for breeding —for example dogs, pigs, cattle, ... whether men can detect fertility in women is debated; recent studies have given ...

www.dogster.com/lifestyle/do-dogs-have-periods-dogs-in-heat-estrous-menstrual-cycle

Jul 17, 2014 ... In short, no, female dogs do not have periods in the same way that human females do. While mammals of all sorts — including dogs and ...

dogsaholic.com/care/female-dogs-get-periods.html

Do Female Dogs Get Periods: A Short Guide to Female Dogs Heat Cycle ... Smaller dogs tend to have their first cycles at an earlier age. How often does it occur?

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 ...

pets.thenest.com/long-bleeding-phase-last-dogs-heat-4132.html

If you have a female puppy, you'll need to be prepared for her estrus cycle. ... Your dog can come into her first heat any time between 6 and 24 months.

www.canidae.com/blog/2010/07/how-to-care-for-female-dog-in-heat

Jul 8, 2010 ... Female dogs that are not spayed go into season or into heat ... then we will have to wait until her cycle is over before she can be spayed.

.ask.com/youtube?q=Do+Female+Dogs+Have+Periods%3F&v=aB14JjvRfgU
Nov 15, 2011 ... this was last thursday. we found out that she was on her period and she was getting blood everywhere so we put a diper on her... she didnt like ...

www.quora.com/My-puppy-just-started-her-period-what-should-I-do

Thank you for the A2A. There are a number of versions of what amount to dog diapers on the ... Male dogs can smell her scent and will do anything to get at your female in heat, including becoming aggressive toward you. ... 'period' the way humans have a menstrual period—that tells me she's about six months old, or more.

herepup.com/do-dogs-have-periods

Even though dogs have similar reproductive organs as other mammals including humans, dogs do not have periods in the same manner that female humans ...