The best way to know when your female chihuahua is in heat is to use a fertility monitor. This is a small device that is attached to your dog's collar or harness. It uses a pheromone to measure your dog's level of fertility. This can help you to know when your female chihuahua is ready to mate. If you see that your female chihuahua is in heat, be sure to wait until she has cooled down. You need to be sure that your female chihuahua is not pregnant before you can mate with her. If she is pregnant, you will need to wait until after she has given birth to the litter.