The symptoms of a dog's first heat are the same as any other heat and include swelling of the vulva and bloody vaginal discharge. Female dogs may urinate more frequently when in heat or develop other behavioral changes.
Although the swelling and discoloration of the vulva is the first visible symptom of heat, it may not be obvious in all breeds. Many owners do not notice it, so the appearance of vaginal discharge is often regarded as the first sign. The discharge is usually very red and bloody at the beginning of the heat, but it thins out and becomes pinkish or yellow as the cycle progresses.
When dogs are in the first part of their heat cycle, they may become uncomfortable or nervous and easily distracted. They usually show no signs of wanting to breed and generally chase off males that approach them. They may become more clingy or affectionate with their owners.
When she begins to ovulate, the dog generally begins to hold her tail out to the side in a behavior known as flagging. She becomes receptive to male attention at this stage. She may be more prone to wandering, and owners may notice stray males approaching or marking their property. The female dog must be kept away from intact males because she can get pregnant even on her first heat.