Author contributions
HJZ and YBG designed the research and drafted the manuscript. MXS
competed data analysis. WWZ, CLF and HYZ performed data collection and
literature search. YS and HXY were chief director of maternal and foetal
unit, and participated in its design and coordination. HXY provided
substantial edits and additions to the manuscript. All authors have
accepted responsibility for the entire content of this manuscript and
approved its submission.