Temporal changes in the distribution of population risk factors attenuate the reduction in perinatal mortality

Changes in the distribution of birthweight, maternal age, and plurality over the study period attenuated the observed reduction in perinatal mortality. It is important to consider differences in the population prevalence of such risk factors to make valid geographic or temporal comparisons. Reasons for the secular shift in birthweight and the implications of any contributing change in obstetric practice require further investigation.