Funding for this research was provided by:
National Natural Science Foundation of China (81771537, 81571504)
Natural Science Foundation of Jiangsu Province (BK 20181117)
Text and Data Mining valid from 2019-11-08
Received: 26 April 2019
Accepted: 30 September 2019
First Online: 8 November 2019
Ethics approval and consent to participate
: This is a retrospective cohort study that did not require informed consent, which obtained ethics board approval from the Reproductive Medical Ethics Committee of Nanjing Drum Tower Hospital. This study followed the ethical guidelines laid down in the 1975 Declaration of Helsinki.
: Not applicable.
: The authors declare that they have no competing interests.