While a pregnant dog's stomach may not visibly grow until about 40 days after conception, your veterinarian can perform a blood test to confirm the pregnancy much earlier. This is because a pregnant dog produces high levels of a hormone called relaxin, which can be measured by a blood test within a few weeks of breeding to detect the pregnancy.
Testing for Pregnancy
Relaxin is produced by pregnant dogs as their bodies develop placenta, and it can be measured relatively early in the pregnancy. High levels are generally detectable as early as day 28, though your vet may be able to confirm the pregnancy via blood test as early as day 22. Alternatively, your vet may be able to detect the pregnancy by performing abdominal palpitations -- like the blood test, this is generally reliable around day 28 of the pregnancy.