aDepartment of Obstetrics and Gynecology, Harbin Obstetrics and Gynecology Hospital, Harbin Medical University (the Red Cross Center Hospital of Harbin)

bHeilongjiang Armed Police Hospital, Harbin, Heilongjiang Province, China

Correspondence to Xiaoli Liu, Harbin Obstetrics and Gynecology Hospital, Harbin Medical University (Harbin Red Cross Center Hospital), No. 415 Xinyang Road, Daoli District, Harbin, Heilongjiang Province, 150070, China. E-mail: [email protected]