Challenges in measuring changes in health and social indicators over time

Robert W Platt1, Jennifer Zeitlin2
1 Departments of Epidemiology, Biostatistics and Occupational Health and of Pediatrics, McGill University, Westmount, Canada
2 INSERM, UMR, Epidemiological Research Unit on Perinatal Health and Women's and Children's Health, , Paris, France
Journal of Epidemiology and Community Health 2009 - April 2009 (Volume 63, Number 4).

Website: http://jech.bmj.com/cgi/content/full/63/4/267

“……The paper by Mortensen et al1 raises important questions in the study of time trends in fetal growth and other exposures and outcomes.

Fetal growth is usually measured by birthweight for gestational age, and small for gestational age (SGA), typically defined using a percentile cut-off point of the weight for gestational age distribution, is a commonly used outcome in perinatal epidemiology studies.2 SGA is also considered an important risk factor3 4 in studies of perinatal exposures on child and adult outcomes, and therefore implicitly an important intermediate between prenatal exposures and morbidity and mortality. Defining SGA requires reference to a population standard to identify the percentiles. Ignoring the potential for bias that arises using a live birth standard,5 there are several choices of population standards that may have an effect on the study.

The primary choice is whether to select an absolute or a relative reference. An absolute reference, where all births are compared with the same reference population (either internal or external), implies that SGA is a fixed construct, and that every baby of the same weight and gestational age has the same level of risk regardless of the population to which they belong. A relative reference, where the reference population is different for different groups, conversely, implies that SGA is only relevant relative to other members of a defined population…..”

Time is on whose side?
Time trends in the association between maternal social disadvantage and offspring fetal growth.
A study of 1 409 339 births in Denmark, 1981–2004

L H Mortensen1, F Diderichsen2, G Davey Smith3, A M Nybo Andersen1,4
1 National Institute of Public Health, University of Southern Denmark, Copenhagen, Denmark
2 Department of Social Medicine, Institute of Public Health Science, University of Copenhagen, Centre for Health and Society, Copenhagen, Denmark
3 Department of Social Medicine, University of Bristol, Bristol, UK
4 Epidemiology, University of Southern Denmark, Odense, Denmark

Abstract: http://jech.bmj.com/cgi/content/abstract/63/4/281

Background: Fetal growth is highly socially patterned and is related to health across the life course, but how the social patterns of fetal growth change over time remains understudied. The time trends in maternal social disadvantage in relation to fetal growth were examined in the context of a universal welfare state under changing macroeconomic conditions over a 24-year period.

Methods: All births in Denmark from 1981 to 2004 were included, and the association between maternal social disadvantage and birthweight was examined for gestational age z-scores over time using linear regression.
Results: All measures of social disadvantage were associated with decreased fetal growth (p<0.001), but with considerable differences in the magnitude of the associations. The association was strongest for non-Western ethnicity (–0.28 z-score), low education (–0.19), teenage motherhood (–0.14), single motherhood (–0.13) and poverty (–0.12) and weakest for unemployment (–0.04). The deficit in fetal growth increased over time for all associations except for unemployment. Also, the measures of social adversity increasingly clustered within individuals over time.

Conclusion: Maternal social disadvantage is associated with decreased fetal growth in a welfare state. Social disadvantage is increasingly clustered so that fewer pregnancies are exposed, but those exposed suffer a greater disadvantage in fetal growth. The economic upturn in the last decade did not appear to weaken the association between maternal social disadvantage and decreased fetal growth.