This study analysed the relative contributions of three possible determinants to the high sex ratio among newborns in rural China - under-reporting of female births, abortions of female fetuses and excess early female neonatal mortality. A cohort of 3,697 pregnancies collected at village level in 20 rural townships from a county in Anhui province in 1999 was followed from pregnancy registration to seven days after birth. The cohort was later completed with 267 retroactively registered pregnancies. In the original cohort, the sex ratio at birth was 152 males to 100 females and in the supplemented cohort 159 males to 100 females, being similar to the sex ratios in the census data of the same townships. The risk of death for girls was almost three times that for boys during the first 24 hours of life. A comparison of the estimated number of missing girls by parity and pregnancy approval status to the recorded abortions and stillbirths suggests that selective abortions of female fetuses contributed most to the extremely high sex ratio among newborns. The under-reporting of female live births and neglect or poorer care of female newborn infants seemed to play a secondary role. New technology has helped the one-child policy to become, in practice, an "at-least-one-son" practice.