Funding for this research was provided by:
Health and Family Planning Commission of Shenzhen Municipality (grant No.201601045)
Received: 26 September 2018
Accepted: 30 May 2019
First Online: 10 June 2019
Ethics approval and consent to participate
: This study was approved by the Institutional Ethics Committee of the Shenzhen Maternity and Child Healthcare Hospital. Written informed consent was given by the caregiver of children for their clinical records to be used in this study.
: Not applicable.
: The authors have no conflicts of interest to declare.