I know very little about dog seasons! Human ovulation etc, yes! Been there done that!!

I know that girls bleed/spot for 10 days and are receptive to male attention for the next 10 days. But my question is this!
Is it given that a dog in season who spends all her time with a male dog will become pregnant or is it a hit and miss affair like human reproduction? I only ask as the potential mum to be has just come into season and wondered if my excitement is a little premature!
Could it not actually happen?
I would ask my breeder but don't want to hassle her too much until we're on the home straight!!
