According to PetMD, phantom pregnancy in dogs, also known as false pregnancy or pseudopregnancy, is most likely caused by hormonal imbalances, especially of progesterone and prolactin. The female dog starts showing pregnancy symptoms or begins lactating within a month or two after her latest heat cycle (estrus) concludes.
PetMD points out that symptoms include behavioral changes such as mothering activity, nesting, self-nursing, depression and restlessness, as well as physical changes such as an distended abdomen, enlarged mammary glands, vomiting, loss of appetite and a watery fluid being excreted from the mammary glands. The symptoms may last up to a month. Occasionally female dogs will have a phantom pregnancy within a few days after they have been spayed. Veterinary treatment for this condition is usually not necessary unless the dog is in undue distress. Symptoms should subside within two or three weeks.