My 12 week old pup has a lump on his tummy which seems to be getting bigger, the breeder said it was where his umbilical cord was.

The vet never noticed it when he had his examination before his jabs.

Should I be worried?

Can it be repaired while he is being castrated? but that won't be for another 2 months!!

Sometimes they don't need to be repaired, I've known quite a few dogs who've lived with a hernia all their life with no probs BUT sometimes they do need repairing, our Chloe had a hernia from when we got her(breeder made us aware but it didn't bother us) and when she was about 4/5 months old we had to have it repaired as she'd torn it internally and it had swollen up and gone purple.

Just my experience, if I was you I would be inclined to leave it as it may never be a problem, just keep an eye on it

HTH
Emily

Some hernias do cause problems - others don't.  One of our puppies had a hernia -about the size of a 10p piece - at 6 weeks old.   She was the only one of the litter who was born breech, and the only one whose cord I cut - and I do think that had something to do with it.

However, when she went to her "forever" home, she went with a caveat on her contract that she did have a hernia which would mean that she should never be bred from, and also that we, her breeders, were fully prepared to pay the additional cost of repair of the hernia when she was spayed, or would pay for it if it became necessary before she was spayed.

When she was spayed, the vet saw no need to repair it as it had all but closed :)

Margot.

Fagan had a henia as a pup and its practically gone now and never caused him any problems.  We were advised to push it in every now and then as this would encourage it to go inside.

Aside to this, 5 months is very young to have a puppy castrated, I personally would wait until he is full mature if it is something you feel you have to do at all.

Thanks for the advice on the hernia. I'll wait and see for now.