Now I've actually achieved a couple of matings I need to go back and re-read the Book of the Bitch as it's been a few years! Just to check though, the eggs do get released more-or-less at the same time don't they? Someone said you shouldn't have too many matings spread over several days or the younger puppies will be born too early compared to the older ones, and I was fairly sure this was an old wives tale and you don't get foetuses that are 6 days younger than the oldest ones if you (for example) have 3 matings spread over 6 days? I'd quite like to keep trying my girl every 2 days as long as she will stand for the dog, to be sure I've got the best chance of having hit ovulation, as I didn't get any tests this time round.

the eggs are released immature over about 36 hours.

They can't be fertilised for 48 hours and will stay viable for up to another 48 hours.

The fertilised eggs then float until about 21 days when they implant, and placentas develop, all together, and further development continues.

So puppies can't be much more or less mature than littermates, due to how long the eggs remain viable (at most 48 hours) and differences in growth are down to placement and placental competence.

That's about what I thought, thanks!