Hi
I've just noticed that my 9week lod lab pup is ever so slightly overshot (think thats the right way round...the bottom teeth are slightly behind the top teeth!).  I did check them when I chose her, but I didn't notice it then, so I'm a little concerned that it may get worse as she grows.  Or do they change again once thay loose their milk teeth?  Both her sire and dam did have perfect scissor bites.  Am I worrying about nothing?
I'd be grateful for any advice.
Thank you!

Stop worry about your baby.  :-)  Top and bottom jaws grow at slightly different rates, it will be some months before they are settled.   By which time you will have forgotten you were even concerned.

Bottom teeth slightly behind top teeth is fine - that is a scissors bite.  The front teeth should not sit on top of each other, otherwise they would grind each other down and the shorter back molars would not touch, which they need to do for chewing.   A slight gap is normal and necessary - it's only when there is a big gap, which you would notice without close inspection, that there is a real problem.  

Unless it's a breed-specific trait, bottom teeth protruding in front of the top teeth is more of a problem.

Stacey

Don't worry that is how it should be and the teeth in the jaw (the lower part) should finish up behind the top teeth but just touching, so when the dog eats the front teeth act like scissors.

Thank you for the advice...you've put my mind at rest!  The gap between them isn't large, I was just a little concerned that it may cause discomfort at a later stage. 
Is the problem hereditary or is it something that just develops that way?

IMO both, if you continually bred undershot to undershot you would have a good chance of 'fixing' an undershot jaw in your lines. On the other hand within a litter of pups you can get the odd pup with a problem that no one remembers having seen before.

Because of the way and order the bones in a puppy grows some strange things can happen, i.e. the bum high stage and the lower jaw going from undershot to scissor or level to overshot.

There are some breeds (mostly toy) where the breeding for some other feature has led to the jaw being incorrect in the majority of the dogs bred and sometimes also missing teeth. When this happens it then becomes acceptable and in the end no ones seems to notice.