Diestrus
Syberian Husky, by huskyboy
The period following estrus is called diestrus, or some sources refer to it as metestrus.
There are two outcomes for diestrus in the dog.
If mating occurred and fertilization was successful, the dog will be pregnant and will enter gestation as her diestrus.
If fertilization was not successful or if mating did not occur, the dog will enter a pseudopregnant state. The reproductive system undergoes very similar hormonal changes whether or not the dog got pregnant. This is a unique aspect of dog reproduction.
The reason this happens is that following estrus the reproductive tract becomes under the control of progesterone. This is the hormone that is required to maintain pregnancy.