Just out of interest and so as not to go off topic on pinkdog's post. This may be a stupid question but if the dates you mate your bitch are 9 or 10 days apart is it possible for there to be puppies present from both matings (ie can ovulation take place over this length of time)? If it is possible and the puppies were born at the earlier date would this cause a problem for those from the later mating as they would be more than a week early?

I don't and never have bred dogs so I have no idea if this is a possibility or not?

All the eggs are released at the same time and live for several days so it depends on how long the sperm take to find the ripe eggs as to how long a period of fertilization took place, I have seen pups which look much less mature than their litter mates but the mating period was only over 48hrs so no reason really why this would be.

You won't get 10 days of a difference in egg fertilization as the eggs don't survive that long.

We mated a Boxer bitch every other day for as long as she would stand for him (on the stud dog owner's advice!) - NEVER AGAIN.  When the litter was born (c section), it looked like puppies from two different litters, three or four of them didn't have the same coat as the others, especially the hair on the top of their heads and just looked as if they should have waited a few more days.  They did catch up and now nearly 14 years later one of the girls is downstairs as I type this.

Ovulation is spontaneous and occurs as a result of a rise in luteinizing hormone.  LH surges over a 24 -48 hr period and this stimulates ovulation. 

Phyllis Holst DVM  says "It has now been firmly established that all the follicles ovulate within a short period of time"  and she further states "ovulation occurs over a period of 24 hours" which I take to imply that from start to finish 24 hours elapses.

Therefore based on this information no particular puppy in a litter can be more than 24 hours younger than its littermate regardless of the timeframe for mating.

I have had litters where some of the pups look bald & pink across the skull (actually looks like sparse translucent hair) from their brow to Occiput and others are fully 'clothed'. 

I've always thought that the "bald" ones are 24 hours younger but the 24 hours occurs at the point where the litter as a whole have delivered at the earliest viable gestational period post ovulation so the difference in coat development is obvious. E.g. a couple more days into gestation the 24 hour age difference between one or more pups wouldn't be detected by a sparse coat on one or more pups.

That's exactly what the vet told me!

Say ovulation occurs on day A-B (24 hour period), the eggs are (we're told) ripe for fertilisation two days later (day C-D). A mating on day C would fertilise most of the early-ripening eggs, and a mating 3 or 4 days later might catch a few remaining ones; we know this happens because of mixed-sire litters, when the misalliance was known to have occurred several days after the offical mating. So fertilisation can occur over several days despite the ova being released over a much shorter time frame.

Would you not consider that in the case of misalliance (with different breed and/or colour of dog) that the sperm deposited by the intended sire is sitting waiting for the ova to mature?  When the mating with the unintended sire occurs, his semen arrives to find mature ova and both dogs contributions randomly find their way to an ovum resulting in a mixed litter.

Mating 3 - 4 days later wouldn't catch later ovulated eggs as such, as supposedly all the eggs are ovulated within a 24 hr period.  Therefore, if there were only 2 eggs (for simplicity) egg 1 would be ovulated and take 2 days to mature (total of 48hrs from ovulation), egg 2 ovulated 24 hrs later so would be mature 48 hrs after, the difference between them is still 24 hours :-)

>When the mating with the unintended sire occurs, his semen arrives to find mature ova and both dogs contributions randomly find their way to an ovum resulting in a mixed litter.

Having seem a few litters like this, with a reasonable-sized litter of 8 or so puppies, there are fewer pups sired from the later mating; I particularly remember a litter of 8 collie-cross pups with one labrador-cross sibling, where the later mating was with the labrador. It would appear that the first mating succeeded in fertilising most of the eggs beforehand.

Yes, I think the first deposit of sperm would have more success in achieving fertilization provided they hadn't been deposited way too early (& were perishing) simply because the sperm would be sitting lying in wait (tucked into the crevices of the fallopian tubes) for the ovulated egg to undergo division/maturation.  The second supply of semen (other sire) would have to figuratively speaking fight their way to the front of the queue. When I said randomly, I meant take pot luck as to which sires sperm actually was successful in penetrating the ova.