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

https://doi.org/10.1016/j.jclinepi.2005.03.015Get rights and content

Abstract

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.

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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