1. Ota ring experience;Wagatsuma,1985
2. Mechanism of action, safety and efficacy of intrauterine devices;World Health Organization;WHO Tech Rep Ser,1987
3. Ein Mittel zur Verhütung der Konzeption;Richter;Dtsch Med Wochenschr,1909
4. World contraceptive use 2005.;United Nations, Department of Economic and Social Affairs, Population Division,2006
5. Skewed contraceptive method mix: why it happens, why it matters;Sullivan;J Biosoc Sci,2006