The canine reproductive system is different than that of a primate. Primates have a menstrual cycle, while sexually mature bitches typically have an oestrus cycle, otherwise known as a season, every six to eight months. During the oestrus cycle, the bitch's body becomes ready for impregnation. During this phase of the cycle, excess hormones can cause the dog's body to function as if it were carrying a litter. This phenomenon is known as phantom pregnancy.
Although phantom pregnancy is not life threatening and typically subsides when the hormonal balance normalises, dogs may show signs of depression and lethargy. Any behaviour that is normally associated with pregnant bitches is normal during a phantom pregnancy, including signs of labour.
During phantom pregnancy, the dog's hormone imbalance causes its mammary glands to become active and begin to produce milk. When this occurs, the nipples become swollen and may become tender. The duration of lactation depends on the severity of the hormone imbalance and the dog's sensitivity to its hormone production. In some severe cases, when the dog is in discomfort, a veterinarian may have to manually milk the teats to relieve the pain.
The canine's body responds to phantom pregnancy in the way it would to a normal pregnancy. The excess hormones that are produced tell the body to prepare to carry a litter. This can result in abdominal distension. The bitch may take on the appearance of a fully pregnant dog, and the stomach may expand.
During a phantom pregnancy, just as with a normal pregnancy, nesting begins toward the end of gestation. Typically, bitches will prepare an area where they feel safe to give birth–away from noise, foot traffic and perceived danger. It is common for bitches to use their bed for nesting, and they may carry cushions, blankets and even clothes to the nest to make it more comfortable. Due to the imbalance of hormones during a phantom pregnancy, the nesting instinct is unpredictable and may occur significantly earlier than it would during a normal pregnancy.
A bitch experiencing a phantom pregnancy may develop maternal instincts. This often causes her to mother inanimate objects such as teddy bears, shoes or clothes. If you find objects that are easily carried in a dog's mouth in the dog's bed or any area that could be used for nesting, it is a sign the dog is mothering. The bitch may position herself among these objects and in areas where foot traffic is high, such as a doorway, in order to protect them from harm.