1. 1. A. Beck "Obstetrical practice ." Baltimore: Williams and Wilkins Co.
2. 2. H. H. Curtis "Obstetrics and Gynecology ," p. 94 , Philadelphia: Saunders.
3. 3. J. B. Lee "Principles and Practice of Obstetrics ," 7th ed. , p. 345 . Philadelphia: Saunders.
4. 4. J. M. Munro Kerr "Combined Text book of Obstetrics and Gynaecology ," 3rd Ed. , p. 410 , Edinburgh: Livingstone.
5. 5. H. J. Stander "Williams Obstetrics ." 8th ed. , p. 473 . New York: Appleton Century.