My friends dog has just been mated. She is a small terrier breed and VERY VERY lively. She makes most border collies look sedate. She does agility and will fetch a ball at top speed for hours if she can get someone to throw it for her.  She plays tug with real determination and never gives up.

My friend wants to know  if she should try and limit the play/exercise her bitch does. Will strenuous exercise stop the puppies developing?  This  is a much wanted  litter and she  wants to do the best she can to ensure  the mating  takes.

I keep my labs having their  normal exercise until the last week or two but  they are nothing like as active as this girl. My gut feeling would be to limit her a bit but just wondered if anyone else knew for sure  as she will get frustrated if she  doesn't get an outlet for her energy

I would not limit her exercise in any way other than not encourage her to be manic in the last Weeks. 

In other words don't throw the ball   I would also limit her running about with larger dogs in case she gets mown down/splatted.

She may be surprised and find their bitch looking after herself like most seem to do, and then you can take your cue from them.

Unfortunately it is not as simple as not throwing the ball as she bounces up and down backwards three feet in the air in front of you if dont throw it and she doesn't give up.
My friend doesn't have larger dogs so she wont be running with them.
What about training classes. I have said I thought she should stop taking her to agility until she  knows whether or not she is pregnant,  but I don't know if I am just being over cautious.

I would certainly stop the Agility, and as with my Friends collie if you ignore them and keep walking on to them they will give up on expecting the Bail, though I suppose it is easier to step lightly onto the toes of a collie than a terrier.

I'm lucky none of mien would chase a ball unless even if you paid them in very yummy treats, well maybe only once or twice.