Affiliation: | a Department of prenatal diagnostic center, Shijiazhuang gynaecology and obstertrics Hospital, Key Laboratory of Maternal and Fetal Medicine of Hebei Province, Hebei, Shijiazhuang, P.R. China;b Department of obstetrics and gynecology, Shijiazhuang gynaecology and obstertrics Hospital, Hebei, Shijiazhuang, P.R. China. |