1. G. H. Arthur(1975 ) Veterinary Reproduction and Obstetrics , 4th edn. BailliereTindall. London, pp.88 -90 .
2. Real-time ultrasound pregnancy diagnosis in the bitch
3. Late Term Fetal Death in the Dog: Early Radiographic Diagnosis1
4. J. H. Highman, and D. Sutton (1975 ) Obstetric Radiology, in A Textbook of Radiology . 2nd edn. Edinburgh. Churchill Livingstone, p952 .