Weeks 8 through 12 are called the fear imprint period. During this period, any painful or particularly frightening experience leaves a more lasting impression on your pup than if it occurred at any other time in his life. If the experience is sufficiently traumatic, it could literally ruin your pup for life.
During this time, avoid exposing the puppy to traumatic experiences. For example, elective surgery, such as ear cropping, should be done, if at all, before 8 weeks or after 11 weeks of age. When you need to take your puppy to a veterinarian, have the doctor give the puppy a treat before, during, and after the examination to make the visit a pleasant experience. Although you need to stay away from stressful situations, do continue to train your puppy in a positive and nonpunitive way.
During the first year's growth, you may see fear reactions at other times. Don't respond by dragging your puppy to the object that caused the fear. On the other hand, don't pet or reassure the dog - you may create the impression that you approve of this behavior. Rather, distract the puppy with a toy or a treat to get his mind off whatever scared him and go on to something pleasant. Practice some of the commands you've already taught him so he can focus on a positive experience. After a short time - sometimes up to two weeks - the fearful behavior will disappear.
This is a page from the book "Dog Training for Dummies"