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             Scandinavian http://sjp.sagepub.com/ Public Health


Association between social position and congenital anomalies: A population-based study among 19,874
                                           Danish women
            Charlotte Olesen, Nana Thrane, Ann-Margrethe Rønholt, Jørn Olsen and Tine B Henriksen
                 Scand J Public Health 2009 37: 246 originally published online 22 January 2009
                                       DOI: 10.1177/1403494808100938

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Scandinavian Journal of Public Health, 2009; 37: 246–251



ORIGINAL ARTICLE



Association between social position and congenital anomalies:
A population-based study among 19,874 Danish women


CHARLOTTE OLESEN1,2, NANA THRANE3, ANN-MARGRETHE RØNHOLT4,
JØRN OLSEN2,5 & TINE B HENRIKSEN6
1
 Department of Paediatrics, Aarhus University Hospital, Aarhus, Denmark, 2The Danish Epidemiology Science Centre at
the Department of Epidemiology and Social Medicine, University of Aarhus, Aarhus, Denmark, 3Department of Paediatrics,
Herning Hospital, Herning, Denmark, 4Department of Paediatrics, Viborg Hospital, Viborg, Denmark, 5Department of
Epidemiology, School of Public Health, UCLA, Los Angeles, CA, USA, and 6Perinatal Epidemiological Research Unit,
Department of Paediatrics, Aarhus University Hospital, Aarhus, Denmark



Abstract
Aims: To examine the association between maternal and paternal educational level and household income and the risk of
giving birth to a baby with a congenital anomaly in a population of Danish women. Methods: We performed a population-
based cohort study, based on record linkage of data from Danish administrative registries. For each pregnant woman in the
cohort, we described financial and educational resources and congenital anomalies in her offspring. We used logistic
regression to model the association between social position and congenital anomalies. Results: The analyses included all
19,874 primiparous singleton deliveries in North Jutland county, Denmark, from 1991 to 1998. There were 1025 (5.2%)
babies with congenital anomalies. The odds ratios (ORs) for giving birth to a baby with a congenital anomaly showed a dose–
response decline, as the mothers’ educational level increased. Women with less than 10 years of schooling had an almost
three-fold increased risk of giving birth to an infant with a congenital anomaly, as compared with women with more than 4
years of higher education (OR ¼ 2.9, 95% confidence interval ¼ 1.8–4.6). Paternal educational level and household income
were, to a lesser degree, associated with congenital anomalies in the offspring. Conclusions: Maternal educational level,
and to a lesser degree paternal educational level and household income, were associated with the risk of giving
birth to a baby with a congenital anomaly. However, the analysis did not take into account maternal health
status, and the results might be due to differential misclassification or residual confounding.

Key Words: Congenital anomalies, maternal education, social position



Background                                                                               health, including congenital anomalies [1–6].
                                                                                         An increased propensity to give birth to a child
We performed this population-based follow-up
study in order to evaluate the associations between                                      with a congenital anomaly has been reported for
maternal and paternal educational level and house-                                       women with lower social position, as compared
hold income and the risk of giving birth to a baby                                       with better-off women [3,4]. Concerning the risk
with a congenital anomaly in a population of Danish                                      of specific congenital anomalies, studies indicate
women. Such studies are necessary to improve our                                         that the association between social position and
understanding of social position as a possible risk                                      the risk of congenital anomalies is birth defect
factor for congenital anomalies.                                                         specific. Thus, low social position is associated with
  Social position has been associated with a variety                                     increased risk of neural tube defects, orofacial clefts
of health outcomes, related to perinatal and infant                                      and transposition of the great arteries [1,2,6–9].
                                                                                                    ˚                 ˚
Correspondence: Charlotte Olesen, Department of Paediatrics, Aarhus University Hospital, Brendstrupgardsvej 100, 8200 Arhus N, Denmark.
Tel: þ458 949 6806. Fax: þ458 949 6023. E-mail: Charlotte.Olesen@dadlnet.dk

(Accepted 5 November 2008)
ß 2009 the Nordic Societies of Public Health
DOI: 10.1177/1403494808100938




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                                                  Association between social position and congential anomalies            247
On the other hand, there is a reduced risk of tetralogy                education) up to 10 years; (b) advanced schooling
of Fallot among the offspring of women with                            equivalent to 11 or 12 years (second stage of basic
low social position [1]. These studies vary widely in                  education); (c) reference group, vocational education
their measures of social position, making comparison                   and training for 1–3 years (e.g. tradesman, mechanic,
difficult. However, important sociodemographic                         hairdresser); (d) higher education for 1–4 years
inequality in the risk of congenital anomalies                         (tertiary non-researched-based education); and
does exist.                                                            (e) higher education for 44 years (university).
                                                                       Higher education required a minimum of 12 years
                                                                       schooling. Household income (DKK/year) was cate-
Material and methods                                                   gorized as follows: (a) 0–130,000; (b) 4130,000–
                                                                       250,000; (c) 4250,000–370,000 (reference group);
Past reproductive history plays a role when a new
                                                                       (d) 4370,000–490,000; and (e) 4490,000.
pregnancy is planned. This effect on decision-making
may differ among different socioeconomic groups,
and forces of selection related to past reproductive
history could therefore bias comparisons among                         Statistical analyses
women with one or more previous pregnancies.                           We used logistic regression to model the association
Offspring of women with non-singleton births are                       between the socioeconomic factors and the risk of
often born premature. We therefore restricted the                      congenital anomalies. Smoking during pregnancy is
study population to primiparous women with single-
                                                                       associated with an increased risk for pre-term
ton births.
                                                                       delivery and giving birth to a small for gestational
   We included all (19,874) primiparous singleton
                                                                       age (SGA) baby. Pre-term and/or SGA babies are
deliveries occurring in North Jutland county,
                                                                       often admitted to specialized paediatric depart-
Denmark, from 1991 to 1998. We conducted the
                                                                       ments, where the diagnostic procedures may imply
study by linking data from the Danish Medical Birth
                                                                       a higher chance of having a minor congenital
Registry and the Integrated Database for Labour
                                                                       anomaly diagnosed. We therefore adjusted for
Market Research (IDA) using the national Danish
                                                                       maternal smoking during pregnancy and maternal
registry of citizens [10,11]. For each mother in the
                                                                       age, and the risk of certain congenital anomalies
cohort, we obtained data on: (a) financial and
                                                                       differing between boys and girls. The risk of
educational resources recorded for the year before
                                                                       congenital anomalies is birth defect specific (see
the birth; (b) congenital anomaly (yes/no) (out-
come); and (c) smoking habits during the early                         Background). Although the gender of the child may
stages of pregnancy (yes/no), age of the mother at                     introduce differential misclassification merely than
time of birth, and gender of the child (potential                      confounding in the study, it was included in the
confounding factors).                                                  regression model. The overall risk of congenital
                                                                       anomalies among boys and girls did not differ
                                                                       significantly. Including the gender of the child in
Data sources                                                           the regression model is therefore very unlikely to
The Danish Medical Birth Registry stores data on all                   influence the conclusions of this study. We did not
births in Denmark since 1973 [10]. The registry,                       adjust mutually for maternal/paternal educational
updated by midwives and doctors attending deliv-                       level. Thus, the estimated effects of parental
eries, contains variables for maternal age at time of                  education are predictive, not causal. Data on
birth, smoking habit of the mother (yes/no), sex of                    maternal education, paternal education and house-
the child and congenital anomalies (yes/no). The                       hold income for 805 (4.1%), 4412 (22.2%) and
IDA draws data from of the Central Office of Civil                     111 (0.6%) children were missing, and were
Registration, wage registers, tax registers, and social                excluded from the analyses.
benefits registers [11]. For our study, we used
variables for maternal and paternal educational
levels and household income.
                                                                       Results
                                                                       We identified 19,874 primiparous women who gave
Definitions
                                                                       birth in North Jutland county during the 7-year study
The educational level (highest completed or                            period. There were 1025 (5.2%) babies with con-
ongoing) was categorized as follows: (a) basic school-                 genital anomalies. Table I shows the characteristics of
ing (including 9 years of primary mandatory                            the women in the cohort.




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248    C. Olesen et al.
                    Table I. Characteristics of women in the cohort (N ¼ 19,874).

                                                                                                         Smoking

                                                            Number             Maternal age (years)   Number   %
                    Maternal education
                      Basic schooling <10 years             2389               24.1                   1200     50.2
                      Schooling 11–12 years                 4075               24.3                   1537     37.7
                      Vocational education                  8122               26.7                   2076     25.6
                      Higher education 1–4 years            3589               28.4                   563      15.7
                      Higher education 44 years             894                28.3                   132      14.8
                    Paternal education
                      Basic schooling <10 years             2298               25.8                   903      39.3
                      Schooling 11–12 years                 1800               25.1                   568      31.6
                      Vocational education                  8178               26.6                   2046     25.0
                      Higher education 1–4 years            1947               28.1                   325      16.7
                      Higher education 44 years             1239               28.3                   134      10.9
                    Household income (DKK)
                      120,000                              2595               228                    1022     39.3
                     4120,000–240,000                       3732               257                    1286     34.5
                     4240,000–360,000                       6293               260                    1788     28.4
                     4360,000–480,000                       5380               277                    1182     22.0
                     4480,000                               1757               297                    309      17.6

                    Educational level: (a) basic schooling up to 10 years; (b) advanced schooling, equivalent to
                    11 or 12 years; (c) vocational education and training for 1–3 years; (d) higher education for
                    1–4 years; and (e) higher education for44 years. Higher education required a minimum of
                    12 years of schooling. Data on educational level of the mother, educational level of the
                    father and household income of, respectively, 805 (4.1%), 4412 (22.2%) and 111 (0.6%)
                    children were missing, and excluded from the analyses.



Maternal educational level                                                      baby with a congenital anomaly (Table II). Women in
                                                                                the income group 120,000 DKK/year had an
The odds ratio (ORs) for congenital anomalies
                                                                                increased estimated risk of giving birth to an infant
showed a dose–response decline as the mothers’
                                                                                with a congenital anomaly as compared with women
educational level increased (Table II). Women with
                                                                                in the income group4480,000 DKK/year (OR ¼ 1.7,
less than 10 years of schooling had an almost three-
                                                                                95% CI ¼ 1.2–2.5) (data not shown).
fold increased estimated risk of giving birth to an
infant with a congenital anomaly, as compared with
women with more than 4 years of higher education
(OR ¼ 2.9, 95% confidence interval (CI) ¼ 1.8–4.6)                              Discussion
(data not shown).                                                               Women with less than 10 years of schooling had an
                                                                                almost three-fold increased risk of giving birth to a
Paternal educational level                                                      baby with a congenital anomaly as compared with
                                                                                women who had more than 4 years of higher
Women whose partner had higher education had a                                  education.
decreased estimated risk of giving birth to a baby                                 In agreement with our findings, Olsen et al. found
with a congenital anomaly as compared with the                                  that the overall prevalence of congenital anomalies
reference group (Table II). The OR for congenital                               among the offspring of a sample of 11,888 Danish
anomalies among the offspring of women whose                                    women increased with decreasing socioeconomic
partners had basic schooling was 1.1 (95% CI ¼                                  status [3]. Although the results were inconclusive
0.8–1.5), as compared with women whose partners                                 (perhaps because of small numbers), it is noteworthy
had more than 4 years of higher education (data                                 that maternal educational level was an important risk
not shown).                                                                     factor for congenital anomalies. Recently, da Silva
                                                                                Costa et al. conducted a case-control study using
                                                                                data collected by questionnaire and patient record
Household income
                                                                                review among a sample of postpartum women in
High income (4360,000 DKK/year) was associated                                  maternity hospitals in Rio de Janeiro from 1999
with a decreased estimated risk of giving birth to a                            to 2001. They found a higher overall proportion




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                                                        Association between social position and congential anomalies              249
                     Table II. Numbers and odds ratios (ORs) with 95% confidence intervals (CIs) for
                     congenital anomalies in social position groups (N ¼ 19,874).

                                                                    Congenital anomalies

                                                              Number                        %                     OR (95% CI)a

                     Maternal education
                       Basic schooling 10 years               155                           6.5                   1.4 (1.1–1.7)
                       Schooling 11–12 years                  211                           5.2                   1.0 (0.8–1.2)
                       Vocational education                   415                           5.1                   Referenceb
                       Higher education 1–4 years             181                           5.0                   0.5 (0.3–0.8)
                       Higher education 44 years              24                            2.7                   0.6 (0.4–0.8)
                     Paternal education
                       Basic schooling 10 years               123                           5.3                   1.1 (0.9–1.3)
                       Schooling 11–12 years                  97                            5.4                   1.0 (0.8–1.3)
                       Vocational education                   429                           5.2                   Referenceb
                       Higher education 1–4 years             83                            4.3                   0.8 (0.7–0.9)
                       Higher education 44 years              60                            4.8                   0.6 (0.5–0.9)
                     Household income (DKK)
                       120,000                               146                           5.6                   1.0 (0.7–1.4)
                      4120,000–240,000                        175                           4.7                   0.8 (0.7–1.0)
                      4240,000–360,000                        370                           5.9                   Referenceb
                      4360,000–480,000                        257                           4.8                   0.8 (0.7–0.9)
                      4480,000                                69                            3.9                   0.6 (0.5–0.9)
                     a
                      Logistic regression analyses adjusted for maternal age, smoking (yes/no), and gender of the
                     child. bReference groups: women with vocational education, women whose partners had
                     vocational education, and women who lived in households with income4240,000–360,000
                     DKK/year. Data on educational level of the mother, educational level of the father and
                     household income of, respectively, 805 (4.1%), 4412 (22.2%) and 111 (0.6%) children
                     were missing, and excluded from the analyses.




of congenital anomalies among the offspring of                                  Measures of social position, congenital anomalies
9386 mothers with less than 8 years of schooling, as                         studied and analytical approach varied widely
compared to the group of women with more than 10                             between these studies, and comparison of their
years of schooling (OR ¼ 1.22; 95% CI ¼ 1.01–1.46)                           findings is therefore difficult. However, a number of
[4]. This demonstrates that there appears to be a                            measures of social position are associated with the
socioeconomic gradient in birth defects in countries                         risk of congenital anomalies. These associations seem
as different in terms of social position, lifestyle and                      to be highly birth defect specific.
healthcare systems as Denmark and Brazil.                                       Wealth or higher education in itself is unlikely to
   Low maternal education level has been associated                          directly affect the occurrence of congenital anoma-
with an increased risk of having a child with a neural                       lies, but low socioeconomic status may affect preg-
tube defect [1,5,6,12]. Low social position has shown                        nancy outcome through a number of mechanisms.
to be associated with an increased risk of orofacial                         Unfavourable social conditions may influence health
clefts [7–9], although this association seems to be                          by forcing people into poor housing, heavy jobs or
inconsistent [1,13,14]. Studies of all heart defects                         specific unhealthy environmental exposure [19]. Low
combined found no association with socioeconomic                             social position is a marker for unhealthy lifestyle
status [15–17]. However, in a case-control study                             [20–22], and the mother’s social background and
examining the association between socioeconomic                              education may influence her compliance with public
status and specific subtypes of heart defects,                               healthcare services and the use of healthcare facilities
Carmichal et al. recently found that low socio-                              such as prenatal screening [23].
economic status was associated with increased risk                              The strengths of our study are its population-based
of transposition of the great arteries. This finding is in                   design and the prospective nature of data registra-
agreement with results from the Baltimore–                                   tion. The data were collected for administrative
Washington Infant Study [1,18]. On the other                                 purposes and independently of a research question.
hand, low socioeconomic status was associated with                           Thus, bias due to differential recall, non-response or
a reduced risk of giving birth to a baby with tetralogy                      selective participation is unlikely to influence our
of Fallot [1].                                                               results. A limitation of our study is lack of data on




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250    C. Olesen et al.
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