1. Contraception in Great Britain;Riphagen,1985
2. United Nations Survey of contraceptive use,1991
3. Intrauterine contraceptive devices;Tietze,1962
4. Prevention of pregnancy by the Graefenberg ring method: a re-evaluation after 28 years' experience;Oppenheimer;Am J Obstet Gynaecol,1959
5. Clinical studies on intrauterine rings, especially the present state of contraception in Japan and experiences in the use of intrauterine rings;Ishihama;Yokohama Med Bull,1959