The testicles start off in the abdomen just behind the kidneys, and during gestation and with the help of male hormones they enter the inguinal (groin) region shortly after birth.   Normally  the testicles drop into the scrotum anywhere between 10 and 40 days.  However it is difficult to confirm this has happened as they are very small and soft.  Our Vet at the 8 week health examination, checks if  the testes  are in place as a matter of course.   If they are not in place our Vet makes a note of this on the vaccination card, this reminds us to keep checking, and  keeps our owners  informed, it also acts as a reminder for your Vet to re-check. 

There is always a chance that between 2 and 6 months the already descended testes may retract, this can be in response to a muscle ( cremaster muscle) contracting in response to cold, stress or fear.  

Between the abdomen and the scrotum there is an area which closes off at around 6 months,  and when this happens the testes will be blocked from dropping down.  In large breeds it can take longer to close and an undescended testicle may drop up to 12 months later.  

When one or both testes do not descend by 12 months, this is called an ectopic testis or cryptorchidism (meaning hidden testicle).  It is more common for one testicle to be undescended than both.  This could be a genetic disorder and performing this mating again isn't advisable. Please discuss the outcome with your vet, as it may or may not have health implications depending on research of the day.