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Approximately three weeks after you're pretty sure your female has bred, it's generally suggested that a pregnancy be confirmed by ultrasound or CT Scan at the vet. This isn't just to be positive there's a pregnancy, but perhaps most importantly, to establish that it's a healthy pregnancy. A dog usually doesn't show physical signs of pregnancy until more than half-way through it.
For more about symptoms of pregnancy in your dog, check this link http://www.pets.ca/articles/article-dogpregnant.htm
There are really no definitive outward signs early on. Even experienced vets sometimes cannot palpate (feel) a pregnancy even weeks from delivery depending on the type and build of the dog. With some breeds of dog, very experienced owners notice a slight change (enlargement/pinkish color) in the vulva after a successful mating.
Breeders who value their companion as well as want to guarantee the best pup outcome, have the pregnancy monitored at regular intervals throughout. If there's any distress or complication that develops and perhaps a C-section is necessary, no one is surprised and plans can be made. It's not unusual for new breeders to lose entire litters (and even the mom) when a delivery goes bad.
Here are very helpful links about what to expect: