Department of Social Medicine, Institute of Public Health, University of Copenhagen
Department of Psychiatry, HS Bispebjerg Hospital, Copenhagen
Department of Social Medicine, Institute of Public Health, University of Copenhagen, Denmark
Correspondence: Dr Merete Osler, Department of Social Medicine, Institute of Public Health, Blegdamsvej 3, DK-2200, Copenhagen N, Denmark.Tel: +45 35 32 7997; fax: +45 35 35 1181; e-mail: m.osler{at}pubhealth.ku.dk
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ABSTRACT |
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Aims To examine the relationship between birth dimensions and discharge from a psychiatric ward with a depression diagnosisin adulthood.
Method A cohort of 10 753 male singletons born in Copenhagen, Denmark in 1953 and for whom birth certificates had been traced in 1965 were followed from 1969 until 2002, with record linkage for date of first admission to a psychiatric ward that led to a discharge diagnosis of depression.
Results Atotal of 190 men, corresponding to 1.8% ofthe cohort, had a discharge diagnosis of depression. The Coxs regression analyses failed to show any association between birth dimensions (birth weight and ponderal index) and risk of psychiatric ward diagnosis of depression in adult life, before or after adjustment for social indicators at birth.
Conclusions This study does not support the existence of a relation between birth dimensions and psychiatric ward admission for depression in adult men.
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INTRODUCTION |
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METHOD |
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Data sources and variables
Data from birth certificates, including information on date and place of
birth, birth weight and birth length, singleton or multiple birth,
mothers age and marital status, and fathers occupational status
at time of delivery, were manually collected for all members of the original
study population in 1965. In January 2002, the Metropolit cohort was followed
up to ascertain vital status through record linkage with the CRS Registry; if
the person was not alive and living in Denmark, we obtained information on
date of death or date of emigration/disappearance. Information on date of
admission to psychiatric wards (from 1969 to December 2002) and diagnosis on
discharge was obtained from the Danish Psychiatric Central Register. This
register has compiled computerised data on admissions to psychiatric hospitals
and to psychiatric departments in general hospitals in Denmark since April
1969, with coverage close to 100%
(Munk-Jørgensen & Mortensen,
1997). The personal registration number ensured that a complete
history of psychiatric hospitalisation could be established for each cohort
member. A total of 230 boys born as twins and triplets and 393 boys with
missing birth data were excluded, leaving 10 753 cohort members for the study
analyses.
Birth weight was recorded in 100g groups and analysed as a continuous variable and in the three categories 52500g, 25003499g and 53500g. Ponderal index used as a proxy measure for intrauterine growth was calculated as birth weight in kilograms ÷ (birth length in metres)3, and entered into the models in quintiles. The marital status of the mother at time of delivery was treated in three categories: married, unmarried (single, divorced or widowed) and unknown. Fathers occupation, which was recorded in 23 categories, was re-coded into three categories: employee (self-employed and salaried employed), worker (manual and non-manual workers) and unknown.
Diagnoses were classified according to ICD8 (World Health Organization, 1967) during the years 19691993 and ICD10 (World Health Organization, 1992) from 1994. The diagnoses included for this study were manic episode and bipolar affective disorder (code numbers 296.19, 296.39 and 298.19 in ICD8 and F30, F31, F34.0 and F38.0 in ICD10) and depressive disorders (code numbers 296.09, 296.29, 296.89, 296.99, 298.09, 300.49 and 301.19 in ICD8 and F32, F33, F34.1 and F38.1 in ICD10).
Statistical analysis
Associations between birth weight, other covariates and depression were
analysed using Coxs proportional hazards regression models with age as
the underlying time scale. Entry time was age at 1 April 1969 and follow-up
ended at the age of first admission with a diagnosis of depression, death,
emigration or 1 January 2002, whichever came first. The proportional hazards
assumption was evaluated for all variables by comparing estimated 7ln(7ln)
survivor curves over the different categories of the variables being
investigated . ln (analysis time), and by tests based on the
generalisation described by Grambsch & Therneau
(1994). A power calculation
based on the birth weight distribution and estimated number of cases showed
that the study would have adequate power (i.e. >80%) to detect a relative
risk of 2.0 or greater. We performed the statistical analyses using STATA
version 7 (Stata, 2001).
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RESULTS |
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DISCUSSION |
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Strengths of the study
The study population consisted of all male singletons born in a
well-defined area (covering a third of the Danish population) who survived to
the age of 15 years. By using the population registers we managed to obtain
complete follow-up information, and consequently these results are based on
birth and psychiatric admission data for more than 95% of this non-selected
population. We assume that our outcome measure is valid, since it was based on
diagnoses confirmed by a psychiatrist and did not depend on the
individuals ability to answer a questionnaire; we do not believe our
study is subject to the selection biases that might occur when the outcome is
based on self-report from a questionnaire.
Limitations
Birth weight has in this area of research been understood as a proxy
measure of foetal growth. Birth weight is, however, a combined measure of at
least two components: foetal growth rate and gestational age at birth
(Wilcox, 2001). We had no
information on gestational age, but ponderal index has been suggested as a
measure of foetal growth which, in theory, should reflect intrauterine growth
restriction (Joseph & Kramer,
1996). We did not find any clear indication of an association
between quintiles of ponderal index and adult depression.
Depression is more common in women than in men, thus it is an obvious limitation of our study that women were not represented in the data-set. The risk of depression increases with age, and our study will not capture the presumed larger number of cases occurring later in life, although the follow-up covered a period of more than 30 years. On the order hand, depression at younger ages may have risk factors that differ from those of later-life depression. Some cases of bipolar disorder are first manifested and diagnosed as unipolar depression, and since our cohort was relatively young a number of diagnoses of unipolar depression will be changed to bipolar disorder at a later stage; consequently, we decided to analyse the two forms of depressive disorder together. Bipolar affective disorder is the most specific diagnosis, however, and therefore we repeated all the analyses for this outcome. The small number of cases in our study reduces the statistical power, in particular of the analyses with bipolar affective disorder as outcome. However, the number of cases of this disorder will increase as the cohort matures, and at a later stage it will also be possible to make a register-based study of the total population, when the children recorded on the computerised medical birth register (started in January 1973) have become old enough to develop severe depression leading to hospitalisation.
We only had information about affective disorders diagnosed during admission to psychiatric hospital or the psychiatric department of a general hospital. A large proportion of patients with depression are treated solely as out-patients in community mental health centres, in private specialist practice or by their general practitioner. Furthermore, no information was available on possible confounders such as maternal depression.
Comparison with other studies
In the Hertfordshire birth cohort study the relation between birth weight
and depression was examined in the late 1990s among 882 men and women born
between 1920 and 1930; cases were identified by means of the Geriatric
Depression Scale and the Geriatric Mental State Examination
(Thompson et al,
2001). There was a strong association between lower birth weight
and risk of depression in men, but no such relation in women. However, in the
most recent study of 8000 male and female participants in the 1970 British
Birth Cohort, lower birth weight was a significant risk factor for depression
(assessed by Rutters 24-item Malaise Inventory) at age 26 years in
women, whereas there was no association between birth weight and risk of
depression in men after adjustment for potential confounding factors
(Gale & Martyn, 2004). In
these two birth cohort studies psychiatric morbidity was assessed by means of
self-completion scales. Although this approach might be more liable to
misclassification than a register-based assessment of outcome, it might catch
the less severe cases of depression treated in general practice. In two
previous studies cases have been identified through hospital admission
records. An Italian casecontrol study with 41 cases found that patients
admitted to hospital with depression were more likely than controls matched by
gender, age, maternal age and marital status to have been small for
gestational age; cases had also lower mean birth weight, although this
difference was of marginal significance
(Preti et al, 2000).
Further, Brown et al
(2000), in an investigation of
a birth cohort in The Netherlands, found that risk of major depression
requiring hospitalisation was increased in groups of men and women who were
exposed to famine during late gestation in the Dutch Hunger Winter of
19441945.
Interpretation
Our study provides no support for the existence of an inverse relationship
between birth dimensions and discharge from a psychiatric ward with a
diagnosis of depression in adult men. The fact that birth weight has been
related to several unexpected outcomes points towards confounding factors as
an explanation of the association (Weiss,
2001; Lawlor et al,
2004). The lack of association between birth weight and severe
depressive disorders, which is known to be closely related to social
circumstances during childhood, indicates that the relation found between
birth size and other chronic diseases in adulthood is not just a result of
residual confounding by factors related to social position.
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Clinical Implications and Limitations |
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LIMITATIONS
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ACKNOWLEDGMENTS |
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REFERENCES |
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Received for publication May 18, 2004. Revision received October 7, 2004. Accepted for publication October 12, 2004.
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