Female dogs that have not been spayed may at one point exhibit signs of false pregnancy or pseudopregnancy, where it appears that the animal's body begins to change in accordance with pregnancy, despite the fact the dog is not carrying a fetus.
The period of estrus in dogs, commonly referred to as "heat," will occur at a regular interval in dogs that have their ovaries intact. According to VCA Animal Hospitals, signs of pseudopregnancy in dogs usually appear four to nine weeks after the previous heat period. The most common clinical signs include enlargement of the mammary glands with possible milk production, nesting, restlessness or other mothering behaviors such as protective guarding of toys or small objects. Dogs experiencing a pseudopregnancy will often exhibit a decreased appetite without an accompanying loss of weight, lethargy and vomiting.
If you suspect your dog may be pregnant, it's best to take the animal to a veterinary clinic for pregnancy testing. While there are several ways to diagnose pregnancy, there is a blood test which measures the hormone relaxin, which will determine if the pregnancy is real or false. False pregnancies are common and not dangerous, so no further steps will need to be taken. Most often a dog with signs of false pregnancy will stop exhibiting the signs within about 3 weeks.
VCA Animal Hospitals recommends that female dogs are spayed in order to prevent heat, false pregnancy and other health issues.