Article in English: When your female dog is going into heat, her vulva will begin to swell. You should notice that her genital area will appear enlarged and protruding outward. The tip of her vulva should move upwards to make it easier for a male dog to penetrate her. These are all signs that she is in heat.  You can turn your dog over on her stomach to examine her genital area or observe her genital area from behind. You should be able to see her swollen vulva from behind. You should also notice your female dog is spotting around your home, such as on furniture, on her bed, or on any carpeting. The spotting or discharge may appear deep red, milky pink, or white. Your dog will be fertile around 12 days after she starts bleeding, as it gets thinner and lighter.  You may also notice the blood or discharge has a distinct smell. The strong smell is meant to attract the attention of a male dog. Some female dogs will bleed very heavily when they are in heat while other dogs will only have minimal bleeding or discharge. You should also observe your dog’s behavior and pay attention to how often she urinates. If she is urinating in small quantities more frequently, she is likely in heat. Her urine contains pheromones and hormones, which will help let males know she is interested in mating. You may notice that your female dog wants to be let outside more often to urinate than she usually does. This could be a sign she is experiencing the period in her heat cycle when she is most fertile. You should also confirm your dog is old enough to mate. Most female dogs should not breed with a male dog until they have had two to three heat cycles. They should be at least one to two years of age. Certain dogs that are prone to hip dysplasia should be x-rayed before mating. The x-rays should be sent to the OFA for interpretation. Your dog must be age 2 or older or the OFA will not accept the x-ray.  If you are not sure your dog is mature enough to breed, ask your vet. Your vet should be able to tell you if your female dog is ready for breeding.

Summary in Vietnamese:
Kiểm tra bộ phận sinh dục của chó. Kiểm tra máu và dịch. Lưu ý nếu chó thường xuyên đi tiểu. Hãy chắc chắn rằng chó cái đang trong độ tuổi sinh sản.