Pecking Union Medical College, Beijing 100730,China;
National Research Institute for Family Planning, Beijing 100081,China;
National Research Institute for Family Planning, Beijing 100081,China;
Pecking Union Medical College, Beijing 100730,China;
National Research Institute for Family Planning, Beijing 100081,China;
Pecking Union Medical College, Beijing 100730,China;
National Research Institute for Family Planning, Beijing 100081,China;
Shanghai Research Institute for Family Planning, Shanghai 200032,China;
State Family Planning Commission, Beijing 100088,China;