There are many reasons why dogs give birth to deformed puppies. Here are some possible factors:

1. Genetic factors: Genetic factors play an important role in dog deformities. If a dog's parents carry certain deformity-related genes, these genes may be passed on to their offspring, causing deformities in the puppies.

2. Malnutrition: Malnutrition is also an important cause of deformities in puppies. During pregnancy, the female dog needs adequate nutrition to support the normal development of the fetus.

3. Disease infection: If the female dog is infected with certain diseases during pregnancy, such as canine parvovirus, toxoplasmosis These diseases may have a negative impact on fetal development and cause deformities or other health problems in puppies.

4. Drug or toxin exposure: During pregnancy, if the female dog is exposed to certain drugs or toxins, such as chloramphenicol, mercury, etc., these harmful substances may have adverse effects on the development of the embryo. This can lead to deformities in puppies.

5. Environmental factors: Environmental factors may also have an impact on the development of puppies. For example, if a female dog is frightened, traumatized and other stressful events during pregnancy, it may have adverse effects on the development of the fetus.

To reduce the risk of deformities in puppies, it is recommended that parent dogs undergo health examinations and genetic testing before breeding to ensure that they do not carry genes or diseases associated with deformities.