You do know that 6 is getting pretty old to breed her. If she has never had pups, the pelvic bones may not spread as easily as when she was younger and she might require a c-section. Just wanted you to know that.
The first step would be to have him tested for brucellosis. It is a sexually transmitted disease that causes sterility and can cause absorption of pups in females who just contract the disease. Read about brucellosis here:
The second thing to be done and probably should be done at the same time is to harvest a sperm sample and check the mobility and number of sperm the male has to determine his reproductive health.
After he has been checked it will be time to check her. There are a large number of possible causes for female infertility. There is a complex chart of testing to determine what her problem might be. It can be found about 3/4 of the way down the following page.
That chart takes you through a step by step approach to determining her fertility. You can start testing him at any time, but some of her testing can only be done when she is in heat.
Now there are some breeds that just have a very hard time breeding naturally but if you are sure they have achieved a tie, I would expect pregnancy as a result. You might lose one litter to herpes but subsequent tries should not be a problem.
I have to run out for about an hour but will check as soon as I return to see if you have more questions or need clarification.
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