[Li, Lin] Natl Res Inst Family Planning, Clin Med Ctr, Beijing, Peoples R China.
[Xu, Tao; Chen, Feilong] Chinese Acad Med Sci, Inst Basic Med Sci, Dept Epidemiol & Biostat, Beijing, Peoples R China.
[Xu, Tao; Chen, Feilong] Peking Union Med Coll, Sch Basic Med, Beijing, Peoples R China.
[Wang, Ning; Gao, Yiyuan; Li, Xiaoyan] Natl Res Inst Family Planning, Ctr Hlth Qual, Beijing, Peoples R China.
[Diao, Xiyezi; Zhu, Silin] Cent China Normal Univ, Dept Stat, Wuhan, Hubei, Peoples R China.