Women;
and;
Children's;
Health;
Center,;
Department;
of;
Obstetrics;
and;
Gynecology,;
the;
First;
Hospital;
of;
Peking;
University,;
Beijing;
100034,;
China;
National;
Center;
for;
Women;
and;
Children;
Health,;
Chinese;
Center;
of;
Disease;
Control,;
Beijing;
10013,;
China;
University;
of;
Hawaii,;
USA;