From breeding point of view:

The moment you decide to mate your bitch and you will find a suitable dog for your her, it is good that you introduce your dog to a possible dog a week before the expected heat. This will help to relieve stress from both. This meeting is more important when the female will be mated for the first time.

The ideal mating time is 9^th-15^th day from the first signs of heat. At this time the bitch could be ready for mating. If possible, it is good to cover the dog more times in this time period, the percentage of successful gestation will increase.

From the veterinary point of view:

In case you are not sure whether your bitch is in heat (see cases of silent heat), you should not wait for cover or if you have any other doubts, do not hesitate to contact your veterinarian. Your vet may resolve the problem with the heat and pregnancy. Many problems can be explained, up to the microscopic examination of vaginal swab or laboratory - determination of hormone progesterone in the blood, etc.