Once a female dog reaches puberty it begins its regular cycles of Estrus or heat which allow the dog to release eggs and reproduce. Most females have 2 cycles per year. Signs that your dog is in heat include include vaginal discharge which is clear, bleeding and swelling of the vulva. There is increased restlessness and there may be increased urination. You also may notice stray dogs hanging around your house, though the female will not accept mating at this point. This early period of heat called Proestrus, lasts between 4 -15 days depending on the size and breed of the dog along with other factors.
The next stage in the estrous cycle is the actual Estrus/heat where the dog will accept a mate and can get pregnant. Unless you are a professional dog breeder you should spay the dog before before her first heat. Some vets will spay a dog during its first heat. This Estrus lasts between 4 - 8 days and signs include the discharge stopping, ovulation (the eggs are released) and pregnancy is possible. If the dog becomes pregnant the gestation time is 58-63 days. There are no visible signs until the last 3 weeks of pregnancy when the teats will enlarge, the belly will swell and finally the birth will occur.
The 3rd stage of the cycle is called Diestrus and lasts 6 - 10 weeks. Many hormonal changes take place and the uterine walls thicken. Dogs that haven't become pregnant go through 'false pregnancy' where their mammary glands may enlarge and produce milk. They may mother toys, moan or whine, prefer extra attention or extra solitude and will show no interest in mating. The last stage in the cycle is Anestrus and will last for 15 weeks. There won't be any hormonal activity, milk production, or interest in mating. If the dog had become pregnant the birth would have already occurred and the mother will be lactating and engaging in typical mothering behaviour until she is ready to wean her puppies.