A "Heat" cycle lasts about 3 weeks (approx. 21 days) The first week you will notice spotting, little drops of blood (invest in a red carpet!) lol! She will not want to mate during this few days.
The second week the vulva will swell, change shape slightly, and with many dogs the bleeding gets less. But it's at this time you have to be concerned with her getting out of your yard or garden/male dogs getting her scent and either making a nuisance of themselves generally, or actually mounting her. Because during this second week, believe me, she will mate with any male dog she can find, and it may only take a minute or two (if your back is turned...!) -for them to make a "tie" -that is when the mating is successful, the dogs get kind of 'tied' together, penis-to-vagina....for anything up to 20 minutes. If this happens, that's more or less it. She will get pregnant, and there will be nothing you can do about it. So watch out.
The third week, there may still be a little spotting or maybe not. But she won't want to mate any more, and will snap at any dogs who get too serious with her back end!
If you really do want her to mate and have puppies one day, then keep her 'entire' (so long as you are certain you can support a litter of pups with vet's fees and all, and bear in mind there CAN be complications during pregnancy or birth, or even after birth, which could cost you a lot extra in vet's fees)....
But if you don't want to breed her, please do seriously consider having her spayed.
Spaying will protect her against Pyometra which can affect any female of any age, but particularly middle-aged or older females. It can strike without warning in even a very fit and healthy dog. It can kill within hours.
Another thing to bear in mind is that protection against breast cancer in the future is greater if the female is spayed BEFORE her first "Heat" cycle.
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