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For a female dog to get pregnant, the male must do what is called "tie" with her. First he will mount her, she will allow this if it is the right time, and then he actually step over his penis with his back legs, and they will be butt to butt - sometimes this can last for five minutes to even an hour. Do not interfere when they are tied, or ever try to pull apart two dogs that are tied, as you can cause serious injury resulting in death to them both, when they are tied, the male is literally locked inside of the female.
The only time that she can get pregnant is when she is in heat, and it is usually in the second week of bleeding to the third week when she will actually accept the male. You can have your vet check if it looks like she is close to "ready" once she is in heat by looking at the cells from a swab under a microscope. The vet can also tell you if the time has passed.
She will not get pregnant if her and the male do not tie.
Remember the best reason to breed is to better the breed, be sure you have people that want the puppies ahead of time, and that you are breeding dogs that you know are disease free (there are several different tests depending upon the breed that your vet will want to perform to eliminate any potential genetic problems that can be eliminated, like venereal and blood diseases.) Breeding for profit rarely works out, more breeders lose money than make money, and more puppies end up in rescues and shelters because they have no homes to go to because the breeder was not prepared in advance. Be sure you have the pedigrees of both dogs, to look at their lines and make sure you are not breeding dogs that are too closely related. There is nothing more horrible than a litter of sick puppies that could have been prevented with proper testing and preparation by the breeder. It is a myth that females need to have puppies at least once to live a fulfilled life. Be prepared to end up keeping all of the puppies, and pay all of the vet bills if all of the people that said they wanted a puppy don't come through in the end. And be prepared for up to twelve of them. Giving birth can also be a dangerous and sometimes deadly process, some females die giving birth so you need to be prepared for the loss of your female, and for the raising and hourly feeding of several puppies. Breeding is not to be taXXXXX XXXXXghtly, and I just want to be sure you consider all of the possibilities before you get into a situation you are not prepared for. If you have two purebred dogs with great bills of health and solid pedigrees, you still need to prepare yourself for the emergencies, and dealing with all of them, including puppies that don't make it sometimes. I hope that you find my answer helpful, I really want what is best for your dog, and any dogs in the future, and am hoping that you want the same.