Is it possible for someone to advise me on the following.  I had my girl mated 3 times over 5 days with a stud dog.  I've only ever usually had one mating and caught every time, but this was his first time so we decided to do three.  She was scanned last week and confirmed pregnant.  Was also told that the pups are 34 days. She could definitely see 7, but there was a possibility of more as she lost count.  This would tie in with her dates and mean she conceived on her first mating.  However, is there the chance that she could have younger pups in that litter due to having the later matings, or would she have just caught on her first mating.  Would smaller pups have shown up on the scan at that time. Would just like to be prepared for smaller pups if there is the chance of this.  I have a feeling there will be.  Am I wrong. Thank you.

although eggs can be fertilised at different matings/times, they enter the utrine horns at the same time and start to develop together - all pups should be the same average size.

It doesn't work like that, no matter how many mating the eggs are only viable for about 48 hours after being fertilizable, and all the fertilised eggs implant around days 18 - 21 and start developing.

Difference in puppy size and apparent differences in maturity are down to how well the individual placentas function.

AS a bitch ages, with each season, the uterine lining becomes less smooth and provide fewer good placental attachment options.