Once the dog is showing signs of pregnancy, it is critical to be able to tell if the puppy is still in her first trimester. There is no way to tell exactly when a Yorkie is pregnant without a thorough physical exam. Pregnancy symptoms vary widely from dog to dog. Some dogs show none, while others exhibit obvious changes in behavior, appetite, and activity. If you cannot tell whether your dog is pregnant, it is best to put her on a pregnancy diet and watch for any pregnancy symptoms to change over the next two weeks.