Contribution to authorship
CM and LY conceived and designed the study. CM, CJ, CF, JW, WJ, YX and LN was responsible for ultrasound examination and late amniocentesis. YH, CA and ZV was responsible for modern genetic technologies. LY was responsible for data management and statistical analysis. CM and LY drafted the paper, which was revised and approved by all authors. CM and PC contributed by revising the manuscript and providing important input.