*Department of Ultrasound, First Affiliated Hospital of Shenzhen University Health Science Center, Shenzhen Second People’s Hospital, Shenzhen

†Department of Obstetricsand Gynecology, The Affiliated Hospital of Chengde Medical University, Chengde, China

S.X. and L.W.: designed the study. S.X., P.Y., and C.J.: data collection. L.Z. and L.J.: assisted with the data analysis and production of the text. S.X.: edited the document.

The Ethics Committee of Shenzhen Second People’s Hospital reviewed this study for exemption from ethical review and informed consent because the data are publicly available and do not involve patient confidentiality.

This study was sponsored by Grants from the Shenzhen Key Medical Discipline Construction Fund (SZXK052) and Sanming Project of Medicine in Shenzhen (SZSM201612027).

The authors declare no conflicts of interest.

Correspondence: Weizong Liu ([email protected]), and Zhengyi Li([email protected]), Department of Ultrasound, First Affiliated Hospital of Shenzhen University Health Science Center, Shenzhen Second People’s Hospital, Shenzhen 518000, China.