Female dogs reach a period better known as heat or estrus when they are ready to mate with males. Estrogen levels increase sharply and then decrease sharply, and eggs are released from the ovaries. If a dog is not spayed, she will exhibit some out-of-the-ordinary behavior that may mean she's in heat.
One of the first changes that happens during the first week of heat are personality changes due to hormones. The dog may become more clingy or grumpy. Some dogs act completely different; however, some dogs do not have such pronounced personality changes. Nervousness is another symptom.
More often than not, a dog loses her appetite during the first week of heat. In rare cases, dogs can have an increased appetite. VetInfo.com reports cases where dogs begin raiding trash cans. However, it is more likely that a dog will lose her appetite than see an increase.
The vulva is near the pelvic opening below the anus. This symptom varies from dog to dog. Some dogs may have very noticeable swelling, while others may not swell at all. A dog in heat may tuck her tail between her legs. VetInfo.com says this is an act to protect the swollen vulva.
Some dogs begin to urinate more often when they are in heat, so you may notice them scratching at the door to go out more often. Much like humans, this is because the dog's uterus is swelling, putting pressure on her bladder.
After the vulva swelling goes down, the dog will begin waving her tail more often than usual. This is an attempt to wave her scent around in order to catch the attention of a male mate.
For about a two and a half to three weeks a dog in heat will expel a bloody discharge. It will grow heavier as time goes on, eventually going from a bright red to a pinkish-tan color. The discharge will then again turn red again until it tapers off as the heat cycle ends.