People's life styles varied considerably between metropolitan and rural areas. The rural areas remain underdeveloped economically and reserved culturally. Thus two regions representing urban and rural areas in Hubei province have been selected respectively. One is in urban area of Wuhan, which is the provincial capital of Hubei with a population of 7 000 000. The other is in rural area of Macheng, a county located 200 km east of Wuhan with a population of 1 200 000. The study was carried out between August 2003 and June 2004. The participants were selected from four hospitals in the urban area and four health centers in the rural area, respectively. The eligibility criteria for enrollment were as follows: 1) healthy pregnant women; 2) at their third trimester; 3) had at least three routine examinations at these antenatal clinics. A total of 410 women were originally enrolled in the study. Information on study procedures was provided to eligible pregnant women. Each woman signed a consent form at the first time interview, confirming her willingness to participate. Ethics approval was obtained from the local Health Department and the research ethics boards of Tongji Medical College, China