Funding for this research was provided by:
National Natural Science Foundation of China (81770316)
Taishan Scholar Foundation of Shandong Province (2017)
Received: 30 October 2019
Accepted: 11 December 2019
First Online: 17 December 2019
Ethics approval and consent to participate
: The study was conducted in accordance with the Declaration of Helsinki and approved by the local ethics committee of Qingdao Women and Children’s Hospital (No. QFELL-KY-2019-034). All participants’ guardians signed the informed consent.
: Not applicable.
: The authors declare that they have no competing interests.