Correspondence: Dr WW Tu, Department of Pediatrics & Adolescent Medicine, Li Ka Shing Faculty of Medicine, University of Hong Kong, Pokfulam, Hong Kong, China. E-mail: wwtu@hkucc.hku.hk Or Dr M Mao, Key Laboratory of Obstetric & Gynecologic and Pediatric Diseases and Birth Defects of Ministry of Education, West China Second University Hospital, Sichuan University, Chengdu, Sichuan 610041, China E-maih dffmmao@126.com