Background and objective: To examine and quantify the changing contribution of some risk factors to the perinatal mortality rate.
Methods: A population-based retrospective cohort study in Northern England compared stillbirth, neonatal, and perinatal mortality rates by birthweight, maternal age, plurality, and gender between 1982-1990 and 1991-2000.
Results: Atlhough the perinatal mortality rate fell by 20% between 1982-1990 and 1991-2000, the proportion of births in high risk groups (low and high birthweight, older mothers, and multiple births) increased. Standardizing the rates for 2000 to the risk factor distribution in 1982 resulted in lower rates. Changes in the birthweight distribution had the largest impact, particularly on neonatal mortality. Nearly a quarter of neonatal deaths in Northern England in 2000 can be attributed to the shift in the birthweight distribution since 1982, especially to the increase in low birthweight births.
Conclusion: 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.