The earliest possible, practical pregnancy diagnosis is with ultrasound exam. We can detect pregnancy as early as 18 to 21 days after mating with this test.
If an ultrasound is negative at 18 to 21 days we usually recommend repeating it at 28 days. A negative test at 28 days is very highly reliable, but we will, on occasion, recommend repeating the ultrasound at 35 days.
When we diagnose pregnancy we will count the fetuses. This count may be inaccurate for a variety of reasons, including resorbtion or abortion of fetuses. We consider our count highly reliable but not 100% reliable.
A bitch can abort or resorb fetuses at any almost any time during pregnancy. This is uncommon, but we have documented pregnancy with ultrasound in a few bitches, only to find later that the fetuses were gone, and no signs of a problem were noticed by the owner, other than the signs of pregnancy resolving. Resorption of less than the entire litter is also possible, with normal birth of the remaining pups.
Yarmouth Veterinary Center.