Funding for this research was provided by:
Projects of medical and health technology development program in Shandong province (202004110774)
Received: 18 March 2021
Accepted: 21 April 2021
First Online: 28 April 2021
: This study was approved by the Ethics Committee of Shandong Province Maternal and Child Health Care Hospital (reference number: 2020SYY006). The data are anonymous, and the requirement for informed consent was therefore waived.
: This is a retrospective case control study and the data are anonymous, and the requirement for informed consent was therefore waived.
: The authors declare that they have no competing interests.