Hello Marna and board,
"Singleton" litters are not uncommon and it's an even money bet whether a bitch can deliver a single puppy normally or not. Many factors are involved. Whelping history is important. Marna's bitch, I assume is prima gravida (first pregnancy)so whelping history for her won't apply directly, but it's important to know the whelping history of her mother, her grandmother, even her great grandmother if the information is available. If these bitches were able to produce puppies normally then Marna's bitch, in better circumstances could be expected to deliver normally. Another factor would be if there is any primary uterine inertia in the bitches heritage. This includes on the sires side as well. If there is a tendency to uterine inertia anywhere in her pedigree then one has to be aware of the possibility that the bitch may have inherited this tendency.
Size of the puppy is also a factor, as singleton puppies are often much bigger than puppies from a normal litter. If this is the case, sheer size can prevent normal delivery. An X-ray at 8 weeks should tell you how big the puppy is.
The simple anatomy and physiology of a bitch is also a major factor. The bitch uterus is actually in 2 chambers. They have one cervix like other mammals but beyond the cervix the uterus actually bifurcates (splits in two) and forms to long sausage like chambers, that in pregnancy can extend right up under the bitches ribs.
In a normal pregnancy, puppies wanting to get outta there will be tossing and turning, "jockeying for postion" as part of the normal whelping process. This stimulation helps the bitch deliver the puppies. With a singleton litter there is obviously only one puppy in one horn of the uterus and this natural stimulation is lost. If there is primary uterine inertia present there is no way the bitch will deliver that puppy.
I have had 2 bitches that have produced singleton litters. Petra, who had 2 out of three litters as singletons, the third being only 2 puppies. A different sire was used for each litter. All litters were delivered by Caesarian section as Petra had primary uterine interia.
Then there was Agnes, who had 9 puppies in her first litter, followed by 2 singleton litters which she delivered normally with the aid of Oxytocin and Calcium injections. CAUTION: Oxytocin should not normally be used until a puppy has been produced. In this case the Oxy was administered by a vet in a clinic.
Point of interest. Agnes' third litter produced one puppy, who was to become Marimat Olivier Duque, named after our dear webmaster at Boxerworld
So onto my advice, which is only my opinion, but it is borne from experience.
Marna, I would calculate the birth date of your bitches 1 puppy and have her booked in for elective C-Section. Mainly so that the vet is on standby and you don't have to go looking for them. I would take her to the clinic on the day the C Section is booked for and get an assessment done at that time. If she is having normal stages of labour and lloks like she can deliver normally, well and good, but if it looks like there could be trouble you'll already be at the vets where things like Oxy and Ca+ injections can be given as a boost. You'll also of course be there to get the baby out ASAP by C Section if needed.
As for the reason for single puppy despite a normal history, I don't think we have enough information to come up with a reason.
How old is this stud dog? Was the bitch "swabbed" pre mating for vaginal or cervical infection. Is the dog regularly checked for penile infection. Infections in either can kill sperm.
Has he had a recent sperm count? Has the sperm been looked at by a Vet and been shown to be normal, even if the count is OK? For example, is the Ph level of the semen normal. Not too alkaline or too acidic? What stage pregnancy are we at with this bitch? She may have conceived 12 puppies and reabsorbed most of them, you'll never know on that but it does happen.
How many matings were done? I have no faith in pre test kits, Bitches only actually ovulate for 2 -4 hours in an entire heat cycle. Actual slides of the discharge where you look for Cornified Cells (pre ovulation) are of far more value.
It's unfortunate I guess that only 1 puppy has apparently been conceived (Ultrasounds can be wrong too remember) but the reason for this can't be speculated on without some investigation.
Hope this information is of value