1. Department of Obstetrics, Maternal and Child Health Hospital of Tangshan, Tangshan, Hebei 063021, P.R. China
2. Department of Obstetrics, Maternal and Child Health Hospital of Luannan, Luannan, Hebei 063210, P.R. China
3. School of Public Health, North China University of Science and Technology, Tangshan, Hebei 063021, P.R. China