The average heat cycle in a dog (estrous cycle), lasts 21 days and occurs every six months. During this cycle, your female dog may get pregnant.
The symptoms to look for with the start of the heat cycle are bleeding from the vagina, and swelling of the vulva and increased urination. If you have a male dog in the area, he will probably be the first one to notice your female’s changes. The bleeding indicates the beginning of the cycle, but the blood spots may be hard to notice especially in smaller dogs.
Be careful with exercising your dog during this time as even the best trained and behaved female dog will succumb to hormones. You can not trust her off her lead or out of your control. Never leave her outside by herself even in a fenced garden if there is any possibility of male dogs nearby.