Does anyone know if a dog of just under 6 months is capable/likely of getting a bitch pregnant? I went to see my vet about having my boy done when he was 5 months and was told that they wouldnt do it until he was 6 months, I explained to them that my bitch would possible be coming into season during that time, and he almost laughed at me and told me that he wouldnt get the urge or be capable of producing anything at that age. She came into season about 2 weeks before he was done and I did notice him taking an interest and separated them, I mentioned it to the vet again when he went in and was told pretty much the same thing as before, so I didnt really give it another thought. We are now just over 5 weeks on from his castration and I have noticed that she has put on a fair bit of weight, she mainly seems to have expanded around the ribcage, her nipples are are very slightly larger but not huge. She doesnt appear to have any other signs that she might be. I am planning on taking her to see the vet at the beginning of next week, but could do with a bit of advice if anyone can please. She has just turned 2 by the way.

I'm afraid your vet is wrong. A five-month dog pup is often fertile and perfectly capable of successfully mating a bitch, and it is several weeks after castration that fertility ceases.

Hopefully your bitch is just having a phantom pregnancy, but unless you can guarantee that she was never alone with a male dog during her season there is always the possibility that she is actually pregnant.

wouldnt get the urge????? id get another vet!!!!

And charge all puppy costs to the idiot vet who told you it wasn't possible!!!!!! :-(

Good god you sometimes wonder if these vets ever actually studied anything! Of course its possible, as you may have just found out. I really wish vets would just pull their heads out of thier own ar$es and stop spouting rubbish!

My sister has had a similar issue with her vet (at the PDSA) with her cats. She has a six month old male and a very sickly rescued five month old female. They are house cats.
The PDSA wont castrate him until mid December and obviously wont operate on her until she is strong enough and healthy enough to cope. However he's been trying to mount and scruff her and now my sister has had to resort to keeping them in seperate rooms (far from ideal!)
When she approached the PDSA and told them the situation and requesting that the male be done as soon as possible to prevent the female becoming pregnant (shes incredibly tiny for 5 months, little bigger than the size of my 14 week old kitten!), they told her they wouldnt do it yet and that the female was too small to become pregnant anyways!

But whats to say she wont?! Stranger things have happened and you see it on these animal rescue shows all the time. Tiny weak queens giving birth to litters.
And you'd think the PDSA would be wanting to take that risk away, not risk a young cats life and possibly add more unwanted kittens to the world.
The female is black and white, the male black and lets face it, if there were to be kittens out of all this, they will be very hard to home due to their colour anyway. What would the harm be in bringing the castration forward? (Shes been asking weekly about this for the past three weeks)

Back to the OP... Fingers crossed it is a phantom, but theres a big risk its the real deal.
Are the dogs in question of the same breed? If not is she a smaller breed/type to him?
I'd be worried about big pups in a small bitch.

The answer is yes, my first BC was from an accidental mating between a 5 1/2 month old dog & a 10 year bitch who had been mated  in her youth & never had a litter & also who had stopped having visible seasons. My bitch was 7 1/2 months younger than her dad & all three dogs(& the other two puppies)lived into their midteens.

I would have kept them separate even if he had been castrated for over a year-in fact I would never leave an inseason bitch & any dog alone together.

Ask your vet for factual information about when a dog becomes infertile after castration & he will not be able to give you a proven time. The only way to tell is to have a sperm test done

JFYI Kittens can be neutered once they hit 2 kilos in weight & early neutering doesn't have the same effect in cats on the growth plates closure as it does in dogs as they mature much early, males do need to be entire & have a fully developed p*n*s. My Reg was done @ 15 weeks, he was starting to hassle his sister. I had a long discussion with my vets about castrating him as everyone will know on here I'm not for pre puberty neutering in dogs

Thank you so much for all the replies, both dogs are small breeds but the little girl is at the smaller end of standard and the boy at the bigger end, so big puppies is a concern. I am going to be worried all weekend now!! What is the situation with scanning, I have read on here about some vets getting it very wrong with size of litters etc and to be honest I have lost any  trust I had in my vet now. Unfortunately I live in an area where the only other vet is over 30 mins drive away but I am going to ask around about this other vets reputation and start going there if it appears to be better.

Um... round here 30 mins drive is the closest vet !   I'd get to whichever one seems to be the best - ask around ;)  unless transport is a problem ;)

is there anychance that the boy can get hurt at mating at a young age,

There is always a chance that either dog or bitch, or both could get hurt at any age if mating unsupervised ;)  this is why responsible breeders always supervise ;)

but I am going to ask around about this other vets reputation and start going there if it appears to be better.

I think that would be a very safe option, where are all those lovely country vets who knew everything about everything, yet again another vet who knows nowt about breeding, it is not uncommon, infact it seems more the norm, I do wonder what our vets of today are taught at veterinary college, there is something amiss somewhere, do they miss the whole section on dog breeding?

Anyhow, you've found the site with lots of dog folk who have been breeding for years so thank goodness for that. :-)

Scans are notorious for getting the numbers wrong, I usually just double it! :-D  But they are invaluable at spotting a pregnancy and also judging the size of those it can see, so make sure that you get that done, to be honest I would wait until your bitch is approx 5-6 weeks in whelp (if she is) to have the scan that way the pups won't be missed at all.

As already advised a castrated dog will still have live sperm for many weeks or months, I expect that your vet also made you think he was safe to be with your bitch after castration. (It wouldn't surprise me)

Anyhow, the scan is your best bet to check on your girl and then you can go from there, I don't know whether you wish to proceed with the pregnancy if she is, but there is always the injection to terminate upto 6 weeks into the pregnancy, and I would insist on your vet giving that F.O.C if you should want it. For your vets information a pup can impregnate a bitch from the moment it's testicles drop and that often is from 4 months onwards.