NationalResearchInstituteforFamilyPlanning;
Beijing100081;
China;
DepartmentofObstetrics;
XinHuaHospital;
ShanghaiSecondMedicalUniversity;
Shanghai200092;
China;
SunYetsanMemorialHospital;
ZhongshanMedicalUniversity;
Guangzhou;
China;
ShandongInstituteofFamilyPlanning;
Jinan250002;
China;
HebeiFamilyPlanningInstitue;
Shijazhuang050071;
China;
MaternalandChildHealthHospitalofShanxiProvince;
Xi＇an710003;
China;
