2[2]Michael D,Kogan,Joyce A,et al.The changing Pattern of Prenatal Care Utilization in the United States.1981~1995.Using Different Prenatal Care Indices[J].JJMA,1998,1279(20):1623~1628.
3[3]World Health Organization.WHO antenatal care randomized trial manual for implgmentation of the new model[J].Geneva:World OrganizatiOn,2002:1~37.