As the weeks are moving along and I can see how sad it is to have just one puppy and the problems that it can bring- I wonder more about why.
She is from a good breeding line. The scanner, very experienced, said it is often down to the dog but the dog has sired other litters. Not many but no singles or particularly small.
what reasons are there in a breed that is supposed to average 8 and often has many more. I have had 15 (14 surviving)

Could some have been reabsorbed, was she given the chv vaccine?

I agree that she has reabsorbed. I have always believed that all bitches actually conceive more pups than they can cope with.  Some bitches may well reabsorb more than they should.  I don't believe it means she can't have a normal number of pups if bred from again.

The dog doesnt determine how many pups are conceived!

It's the bitch who determines the litter size; the dog releases millions of sperm but the bitch releases the eggs - and the eggs need to be ready at the right time to meet the sperm. A mating too early or too late from the date of ovulation will result in a smaller litter, or no puppies at all. Plus of course there's the possibility of resorption for many reasons.

I know of 3 bitches that had single pups -all ended up with a section and lost the pup. All were mated again and then had normal sized litters.

Well that is good to know and yes she did have the chv vaccine. I thought it was the bitch that determined the number but the scanner said the dog.
What about the effect of too early or too late. I wondered how much account that took or perhaps not at all?

I have always been led to believe it's the bitch that decides the number of puppies and the sire, the sex

> the scanner said the dog.

Well they are not right...it is the bitch!!

Frightening lack of knowledge from the scanner - ok you don't need the biological knowledge to scan, but if you haven't got it you shouldn't be offering an opinion. The bitch releases a small number of eggs, 1 to 12 or 15 in some breeds etc, whereas the dog releases millions (I think it's millions, biology GCSE a long time ago) of sperm. So therefore the bitch determines the number of foetuses. The bitch has 2 X chromosomes and the dog has some sperm with X and some with Y, so the foetuses are either XX female or XY male. So the dog determines the sex. Simple!

Also many more eggs are fertilised than manage to implant.

Successful pregnancy depends on a good endometrial lining, that the placenta can develop into, and the hormones that maintain pregnancy. 

The more seasons a bitch has the less good that lining becomes, so generally litter size can reduce with age.

Things can go wrong with the developing foetus and they die and are resorbed. Causes of fetal death are many.

Surely both dog and bitch determine how many? If the stud has millions of sperm with motility problems then they might not reach more than one of the bitches eggs and fertilize them. Or if they have good motility but poor genetic quality they may reach the eggs but not create a viable zygote. Or say you mated before ovulation but not after, poor quality sperm would die quite quickly, before ovulation occurred. I read in a forum somewhere a person hugely improving her dogs "sample quality" by clipping out around his nethers (heavy coated dog, equatorial location).

Is the singleton a bitch? Are they usually?

This has all been very interesting. The singleton is a dog. Was not particularly large at birth and born naturally and easily. Text book actually.
Was born 2 days after my original due date calculated.

I was asking about the pup's sex because there is a phenomenon in human reproduction which makes babies conceived via intercourse at or immediately after ovulation more likely to be male and those conceived via intercourse a few days prior prior to ovulation to be female.

This is because male sperm (that is, sperm carrying a Y chromosome) tend to swim faster and female sperm (carrying an X chromosome) tend to live longer. In humans sperm can live 4-7 days in a friendly uterus fallopian tube, eggs tend to be viable for only around 24-48 hours after ovulation. I know all this because my eldest was conceived while using a natural form of contraception (which had worked fine for 2.5 years!) and I ovulated either 7 days after or 6 days before the relevant liaison! She is a girl and it's likely that the sinus infection I had that week messed up my cycle.

This is also why ivf is slightly more likely to produce boys than girls (as the egg is ready and close by,  the fastest sperm win the race) and why men with poor motility (not good swimmers) are slightly more likely to have daughters (the male sperm die before they are able to reach their destination).

Nothing to do with dogs, sorry. I'm not sure how much things can vary within similar mammals. I know things are quite wildly different with marsupials. Based on the human side of things I would say I think your girl had a small litter and reabsorbed most of it for some reason.

With mine around the time they implant my girl was in the car when someone drove into the back of ours. She was thrown from the seat and into the footwell and my car written off. She didn't like traveling in the car to begin with so you can imagin how stressed she was. I do wonder if this may have played a part in her only having the one.