[1] WHO Collaborating Ctr Res Human Reprod, Dept Epidemiol & Social Sci Reprod Hlth, Shanghai Inst Planned Parenthood Res, Natl Populat & Family Planning Key Lab Contracept, Shanghai, Peoples R China
[2] Kaiser Fdn Res Inst, Div Res, Oakland, CA USA
[3] Yixing Ctr Hlth Surveillance, Yixing, Peoples R China