Received: 10 April 2018
Accepted: 20 May 2019
First Online: 30 May 2019
Ethics approval and consent to participate
: The Medical and Biological Research Ethics Review Committee of Xi’an Maternal and Child Health Care Hospital approved the study and the statement of informed consent beforehand, reference number: 2011012. All the participants i.e. the parents/guardians of the children consented willingly to participate by signing the written consent.
: Not applicable.
: The authors declare that they have no competing interests.