Original ArticleTemporal changes in the distribution of population risk factors attenuate the reduction in perinatal mortality
Introduction
Perinatal mortality in the United Kingdom has declined over the past 20 years. However, the rate remains higher than that in many other developed countries [1]. Advances in neonatal intensive care have resulted in the prevention of death for many vulnerable babies, but only in its deferral for some. Consequently, extended perinatal mortality, including stillbirths and all neonatal deaths, has been advocated as a better reflection of perinatal events [2]. Further, the separate reporting of stillbirths and neonatal deaths has been recommended, as their etiologic determinants have diverged substantially [3].
Perinatal mortality is affected by a number of risk factors, including birthweight, plurality, maternal age, parity, and infant gender [4], [5], [6], [7], the distributions of which have changed in the birth population over the past 20 years [8], [9], [10], thereby potentially influencing current observed rates of perinatal mortality. Quantifying the impact of these changes is important to understand trends in perinatal mortality and to make meaningful comparisons of perinatal mortality rates over time and between geographic locations. This population-based study examines and quantifies the contribution of changes in the distribution of birthweight, maternal age, and plurality to observed trends in perinatal mortality in the north of England over nearly 2 decades.
Section snippets
Data sources
Data on perinatal deaths (stillbirths and all neonatal deaths) during 1982–2000 were obtained from a long-standing population-based register, the Northern Perinatal Mortality Survey (PMS) [11].The PMS covers the former Northern Health Region of England as defined in 1972 [12]. The study region is a mixture of several heavily populated urban areas and widespread rural communities. It has a predominantly Caucasian population of approximately 3 million (ethnic minorities account for under 2% [13]
Results
There were 686,456 births between 1982 and 2000, including 3,386 stillbirths and 3,168 neonatal deaths. The stillbirth and perinatal mortality rates declined steadily between 1982 and 2000 (Fig. 1).
Main findings
The perinatal mortality rate fell by 20% between 1982–1990 and 1991–2000. Stillbirth and neonatal mortality rates have declined over time in all birthweight categories. However, there was an increase in the proportion both of LBW and of heavy (≥4,500 g) babies: the risk of perinatal mortality remained extremely high for small births and over twofold that for the reference category for the heavy birthweight group.
Of the examined risk factors, the secular shift in the birthweight distribution had
Conclusion
Shifts in the distribution of birthweight, maternal age, and plurality have significantly influenced the observed perinatal mortality rate, such that the perinatal mortality rate observed in 2000 was 14% higher than it would have been had the underlying distribution of birthweight, maternal age, and plurality remained as it was in 1982. After adjustment for the change in the distribution of these factors, the neonatal mortality rate has declined to a greater extent than the stillbirth rate,
Acknowledgments
The study was funded by the Newcastle University Hospitals Special Trustees. We thank Mrs Marjorie Renwick, Operational Manager at the Regional Maternity Surveys Office, and the PMS Steering Group for access to the data, and are grateful to all the district convenors and coordinators in the Northern Region for their continued collaboration and support of the PMS. We acknowledge the help of staff at ONS for providing birth data for the Northern Region.
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