When a dog is in heat, she may be seem more alert, nervous and distracted than normal, and she may urinate more frequently. Her vulva swells, and she is likely to have a bloody vaginal discharge at first, but it becomes straw-colored when she is ready to breed.
Heat, or estrus, occurs when a female dog experiences an increase and then a precipitous decrease in estrogen levels and releases eggs from her ovaries, making her receptive to mating. Unspayed female dogs experience their first estrus when they are between 6 and 24 months of age. First estrus tends to occur earlier in small-breed dogs and later in larger breeds. Female dogs go into heat approximately every six months. Male dogs are attracted to female dogs in heat for about 18 days, but the female is only receptive to males for about nine days.
Having a female dog spayed prevents her from experiencing estrus and unwanted pregnancies. It also reduces her chances of developing breast cancer and diseases that affect the reproductive system. Veterinarians recommend spaying female dogs prior to their first heat cycle. Female dogs can undergo the procedure when they are as young as 2 months old, but a veterinarian can suggest the best time to schedule the spay.