Funding for this research was provided by:
The Social Development of Science and Technology of Shaanxi Province (2019SF-100)
The Bureau of Xi’an Science and Technology (201805098YX6SF32(1))
The Center for Maternal and Child Health, Chinese Center for Disease Control and Prevention (2015FYJ002)
Text and Data Mining valid from 2019-11-19
Received: 7 May 2019
Accepted: 16 October 2019
First Online: 19 November 2019
Ethics approval and consent to participate
: This research was approved by the Ethics Committee of Science of Medical center, first affiliated hospital of Xi’an Jiaotong university (No: XJTU1AF2018LSK-245). It was a database based study, and all data was collected from open access website, so the consent to participate was not applicable.
: Not applicable.
: The authors declare that they have no competing interest.