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Dogs generally have three stages to the heat. The first stage lasts for nine days at which time she will not allow the stud to mate. You will notice swelling and light bleeding.
The second stage lasts for nine days as well and she will be ready for him. She will flag him and stand for him thus allowing the mating. This is the only period she will be fertile.
None days latter she will be going out of heat, the swelling will be noticeable and there may still be slight bleeding but your male may lose interest and she will no longer stand for him.
So if she has been in heat for seven days you may want to give it another two days or so before introducing them.