When a female dog enters puberty, she begins her first estrus cycle. Estrus is when your dog goes into heat, and it's a period during which she is able to become pregnant. When your dog is in heat, her vulva enlarges, and she may leak a bloody discharge from the vagina and mark territory with her urine to advertise her fertility.
Starting Heat Cycles
Female dogs go into heat when they are unspayed and reach puberty. The time at which this happens depends on the breed. While the average age at which female dogs go into heat is about 6 months, smaller dogs may enter estrus earlier and larger dogs may not do so until they are nearly 2 years old. If you don't want your dog to experience heat, you can have her spayed before it happens -- at your vet's discretion, she may be spayed when she is only 2 months old. According to the ASPCA, spaying your dog before she goes into her first heat cycle significantly reduces her risk for tumors of the mammary glands.