This is a fantastic explanation of canine ovulation, by Michelle Weber.
Eggs fertilization conception

• Puppies aren't conceived at different breedings. It is scientifically impossible.
• When a dog ovulates, all eggs are released at the same time. The eggs can not be fertilized at this time.
• It takes 2-3 days for the eggs to ripen. When they are ripe, they can be fertilized. The eggs then die 2-3 days after they ripen and there is no chance of pregnancy at that point.
• If a dog is bred before eggs are released or ripen, sperm will attach to the uterine wall and wait for the signal that eggs are ripe.
• Fresh sperm can live 7+ days inside, fresh chilled can live as much as 3 days, while frozen only has a lifespan of about 12 hours.
• Each ejaculation contains 100's of thousands of sperm if not millions. Put all this together, you realize eggs can only be fertilized over a short period of time anyway. The massive amount of sperm will get all the eggs at the same time.
• The eggs also do not attach to the uterus right away. They float around as little balls of cells for 17-20 days.
• It is not until 3 weeks after ovulation that pups are attached to the wall and develop placentas.
• All this aside, fertization time does not determine gestation in canines. Gestation is counted from ovulation.
• Before people understood canine reproduction, it was believed that more breeding produced more pups and weak pups were conceived later. This is all old wives tales and not scientificly accurate one bit.
• All eggs are fertilized within the same period of time.