Received: 18 October 2016
Accepted: 7 March 2018
First Online: 13 March 2018
Ethics approval and consent to participate
: This study was approved by the Ethics Committee of Jining Maternal and Child Health Care Hospital & Ethics Committee of Shandong province. Written informed consent was obtained from the parents or legal guardians of the children before enrollment.
: Not applicable.
: The authors declare that they have no competing interests.
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