National Research Institute for Family Planning, Beijing 100081,China;
National Research Institute for Family Planning, Beijing 100081,China;
National Research Institute for Family Planning, Beijing 100081,China;
Peking University School of Stomatology, Beijing 100005, China;
National Research Institute for Family Planning, Beijing 100081,China;
National Research Institute for Family Planning, Beijing 100081,China;
Peking Union Medical College, Beijing 100005, China;
National Research Institute for Family Planning, Beijing 100081,China;