1. O'DonnellThomas J. S.J. Medicine and Christian Morality. Alba House/Society of St. Paul: New York. 1976, pp. 203–206.
2. HellmanLouis M., and PritchardJack A. Williams Obstetrics. 14th Edition. Appleton-Century-Crofts: New York, 1971, pp. 1069–1072.