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| Journal of the New Zealand Medical Association,
02-March-2007, Vol 120 No 1250
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Māori and non-Māori differences in
caesarean section rates: a national review
Ricci Harris, Bridget Robson, Elana Curtis, Gordon Purdie,
Donna Cormack, Papaarangi Reid
Caesarean section (CS) births have become increasingly
common in New Zealand; they have risen from 9.6% in 1983/84 to 22.1% in
2001.1–4 Within the context of rising CS
rates, concerns over disparities by ethnicity have also been raised with the
lowest rates among Māori women compared with other
groups.2,3,5–7
In 2001, of women giving birth in hospital, the CS rate for
Māori was 15.2% compared with 24.5% among European
women.4 Pacific women had a CS rate between
that of Māori and European women at 18%.
Lower rates of other obstetric interventions have also been
documented among Māori, including instrumental vaginal delivery,
inductions, epidural analgesia, and
episiotomy.2–4 It has been suggested
that, given higher risk pregnancies and more health problems among Māori
women, higher rates of obstetric intervention, including caesarean section,
might be expected.2 This raises questions about
why there is an apparent inverse relationship.
While differences persist after considering maternal age,
clinical factors (such as parity,5,7 and
non-clinical factors) have been raised as possible contributors to ethnic
differences.2,7
One New Zealand study5
undertaken at National Women’s Hospital (NWH) examined associations
between ethnicity and obstetric intervention (including caesarean section) after
controlling for parity and obstetric risk in more detail. Among 43,367
singleton, cephalic deliveries, not preceded by caesarean section between
1992–1999, results showed that rates of pre-labour caesarean remained
lower for Māori and Pacific women than for all other ethnicities (after
controlling for age, parity, and multiple clinical risk factors). For caesarean
delivery rates overall, however, adjusted analyses were not significantly
different for Māori or Pacific Island women compared to ‘Other’
ethnic groups.
Differences in CS by deprivation have also been documented
in New Zealand with lower rates of CS at increasing levels of area
deprivation.2 As with ethnicity, the authors
suggest that given the likely higher clinical need for intervention among women
from more deprived areas, one might expect higher rates of intervention.
Ethnic disparities in deprivation (with the skewed
distribution of the Māori population towards the most deprived
areas)8 may therefore contribute to differences
in CS between Māori and non-Māori women. The impact of socioeconomic
position on ethnic disparities in CS has not been considered in New Zealand. The
interplay between ethnicity and deprivation requires further consideration in
order to contribute to our understanding of ethnic disparities in caesarean
section.
This study aimed to investigate the relationship between
caesarean sections, deprivation, and ethnicity. It also aimed to examine
Māori/non-Māori disparities in caesarean section after controlling for
possible confounding factors using national hospital information.
MethodNational hospital data with any diagnosis of ICD-9-CM
V27, the code for outcome of delivery, were obtained from the New Zealand Health
Information Service (NZHIS). This includes deliveries in public and private
hospitals. Coding of caesarean sections as elective and acute was introduced in
1996.9 Therefore, data analyses were restricted
to the period 1 January 1997 to 30 June 2001. For any women having a hospital
birth during this period, all admissions up to one year prior to birth, and any
previous hospital births after 1 January 1988 were also obtained.
Total, elective, and acute caesarean sections
(numerator) were analysed by ethnicity and deprivation among all admissions of
women having hospital births (denominator).
Total CS were coded as any ICD-9-CM procedure beginning
with 74.0 or 74.1.
ICD-9-CM defines an elective caesarean as a
caesarean section carried out as a planned procedure before the onset of labour
or following the onset of labour, when the decision was made before labour
(ICD-9-CM codes 7401, 7411).
An acute/emergency caesarean is defined as a
caesarean required because of an emergency situation—e.g. obstructed
labour, fetal distress. It is best described as "when the caesarean section is
performed having not been considered necessary previously" (ICD-9-CM 7402,
7412). Other types of caesarean sections were excluded from the analysis
(n=32).
In this study it was assumed that there was likely
undercounting of Māori in hospital data.10
Therefore, any record that included Māori in either the event or National
Health Index (NHI) ethnicity fields was identified as being of Māori
ethnicity. Where individuals with multiple admissions were ‘ever’
recorded as Māori in any admission record, the ethnicity for all records
for that individual was identified as Māori. All remaining records
(including those with no ethnicity specified) were classified as
‘non-Māori’.
A New Zealand Deprivation Index (NZDep96) scale from 1
to 10 was assigned using women’s NHI meshblock. NZDep96 is an area-based
measure of socioeconomic deprivation that combines (by principal component
analysis) nine variables from the 1996 Census, reflecting eight domains of
deprivation.11 Each variable was calculated as
the proportion of people with the specified deprivation characteristic in each
meshblock in New Zealand. The ordinal scale ranges from 1 to 10 whereby 1 is
assigned to the least deprived 10% of areas and 10 to the most deprived 10% of
areas.
All statistical analyses were conducted using SAS
version 9.1 (SAS Institute Inc, Cary, North Carolina). Caesarean section rates
were calculated as proportions of all admissions of women having hospital
births. Univariate associations with ethnicity and deprivation decile were
examined. Data with insufficient information available to assign a deprivation
decile was excluded from deprivation analyses. Chi-squared
(χ2) tests were used to test for
differences in proportions. Mantel-Haenszel Chi-squared tests were used to test
for significant trends.
Age is an important confounding factor as CS increases
with increasing age, and the age distribution for women having babies is
different for Māori and non-Māori (i.e. Māori women tend to be
younger). Logistic regression models were run to examine the relationship
between CS and ethnicity after adjusting for age, as well as age and
deprivation. Ethnicity was entered into the models as a dichotomous variable
(non-Māori vs Māori). Age and NZDep96 were included as continuous
variables.
Additional models were run on a selected group of
women—adjusted for maternal age, deprivation and other potential
confounding factors. To control for parity and previous caesarean, this analysis
was restricted to women having their first baby—i.e. no hospital delivery
since 1988, and no coding of previous caesarean (ICD-9-CM 654.2).
Other available clinical variables coded on the
admission with delivery included:
In addition,
District Health Board (DHB) of women’s residence was also entered into
models to assess the impact of regional differences in CS on ethnic
disparities.
An interaction term (ethnicity × NZDep96) was
added to the models to test for any differences in the relationship between
deprivation and CS for Māori and non-Māori women. Odds ratios (OR) and
95% confidence intervals (CI) are presented for non-Māori compared to
Māori and for each increasing NZDep96 decile.
All odds ratios presented are for each category of
caesarean section versus all other types of birth—e.g. total CS vs all
else, acute CS vs all else and elective CS vs all else. For acute CS the
comparison group also includes women with an elective CS birth. Models were also
run estimating the odds of acute CS vs no CS. These showed similar results and
are therefore not presented.
ResultsFrom January 1997 to June 2001, there were a total of
243,539 admissions for women having hospital births (21% Māori and 79%
non-Māori). The most common age group (in 5-year bands) for Māori
women to give birth was 20–24 years, and for non-Māori it was
30–34 years. Thirty-two of these births involved other types of CS and are
excluded from the analysis. The CS rate was 19.5% (n=47,363).
Large disparities in area deprivation were evident between
Māori and non-Māori women, although both groups were slightly more
deprived than Māori and non-Māori in the total population.
Caesarean section rates, both elective and acute, were
significantly higher among non-Māori women compared with Māori women
(Table 1). Among women who had a CS, Māori women were significantly more
likely (p<0.0001) to have an acute CS (71% of all caesareans) compared to
non-Māori women (64%).
Table 1. Caesarean section by ethnicity, Jan
1997–June 2001 (number, percentage of deliveries, and non-Maori to Maori
ratio)
There was a significant relationship between CS rates and
deprivation (Table 2), with CS rates decreasing with increasing area deprivation
for both acute and elective CS (p<0.0001, Mantel-Haenzel chi-square test for
trend).
Table 2. Caesarean section by deprivation, Jan
1997–June 2001 (number and percent of deliveries)
Logistic regression modelling showed that after adjusting
for age, non-Māori women were significantly more likely to have a CS birth
than Māori for total, elective, and acute caesareans (Table 3). After
adjusting for age and deprivation, the odds of CS comparing non-Māori to
Māori reduced slightly overall and was mostly due to the stronger
association between deprivation and elective CS. Adjusting for deprivation had
little effect on the association between ethnicity and acute CS.
For total and elective CS there was a significant gradient
by deprivation after adjusting for age and ethnicity, with a decreasing chance
of CS with increasing deprivation (Table 3). This relationship was not
significantly different for Māori and non-Māori.
Table 3. Logistic regression models—odds
ratios of caesarean section among all admissions of women for delivery, January
1997–June 2001
208806 observations, 34701
excluded with missing NZDep96; *Significant interaction;
†linear fit from decile
1 (least deprived) to decile 10 (most deprived).
For acute CS, the results did not demonstrate a significant
relationship with deprivation after adjusting for age and ethnicity (Table 3).
However, modelling with inclusion of the interaction term indicated that the
relationship by deprivation is significantly different for Māori and
non-Māori (χ2=6.17, DF=1, p=0.013).
Among Māori women there was a significant relationship
between increasing deprivation and less likelihood of an acute CS (OR at each
level of deprivation=0.981, 95%CI=0.968-0.994, p=0.0036). Among non-Māori
women, there was no significant relationship between deprivation and acute CS
(OR at each level of deprivation=0.998, 95%CI=0.993-1.003, p=0.49).
The association between ethnicity and CS may be confounded
by other factors such as parity and clinical risk, or differential access to
services. Table 4 presents analyses restricted to women having their first baby
(no previous admissions since 1988) with no previous CS. There were 108,636
admissions (16% Māori, 84% non-Māori).
Among Māori women, the CS rate was 15% compared with
24% among non-Māori. The elective CS rate among Māori was 1.9%
compared with 4.6% among non-Māori. The acute CS rate among Māori was
14% compared with 19% among non-Māori.
Among women having their first baby in hospital,
non-Māori women were more likely to have a CS than Māori, after
adjusting for age. After adjusting for age, deprivation, and other clinical
factors, the OR for non-Māori compared to Māori is reduced towards one
for total, elective, and acute CS—but non-Māori are still
significantly more likely to have a CS birth than Māori, especially for
elective CS.
Table 4. Logistic regression models—odds
of caesarean section (non-Māori [nM]: Māori [M]) among women having
their first baby, with no previous CS; January 1997–June
2001
93374 observations, 15262
excluded with missing NZDep96 or other variables;
‡Model adjusted for age,
deprivation, multiple births, fetal presentation, gestation at delivery, HT,
APH, DM; ‡‡Model
adjusted for age, deprivation, multiple births, fetal presentation, gestation at
delivery, HT, APH, DM, DHB.
There were no significant interactions between ethnicity and
deprivation detected in these analyses.
With the addition of the DHB variable into the models, the
odds of CS for non-Māori compared to Māori are further reduced towards
one, particularly for elective CS. This suggests that the differences in CS seen
across different DHBs may be contributing to the ethnic differences in CS seen
at a national level.
DiscussionOur study shows significant differences between Māori
and non-Māori for total, elective, and acute CS after controlling for
deprivation, with non-Māori women more likely to have a CS than Māori
women. This suggests that while deprivation may explain some of the disparity
between Māori and non-Māori in CS, it does not explain it all. In
addition, lower rates of CS among Māori women persist after also
controlling for available clinical factors. Among women having their first baby
with no previous CS, differences between Māori and non-Māori are
greatest for elective CS.
Strengths of our study are that it explores ethnic
disparities in CS among women having hospital births nationally, and considers
the role of socioeconomic position (as measured by NZDep96) as well as other
potential confounders. However, there are a number of limitations that should be
considered in the interpretation of our findings.
It is likely that there is some degree of misclassification
of variables. We used administrative data, which introduces the possibility of
coding errors with regards to ICD-9-CM classification. Despite our attempts to
minimise the undercount of Māori hospitalisations by the ‘ever
Māori’ method of categorisation, subsequent studies using this method
suggest that it improves but does not fully account for this
undercount.12 Such misclassification applies to
both numerators and denominators in this study, and would therefore tend to bias
disparities to the null.
Misclassification may also occur where record linkage was
used to determine variable classification—e.g. parity. Duplicate or
incorrect NHI numbers have been identified as affecting data
quality3 and may lead us to underestimate
parity if individuals are not accurately linked to having previous births.
Whether this is different for Māori and non-Māori women is unclear.
There may also be residual confounding of the disparities in
CS between Māori and non-Māori. For example, the impact of
socioeconomic position on ethnic inequalities in CS may well be underestimated
in our study as we only used NZDep96. This is only one measure and will not
fully capture all dimensions of socioeconomic
position.13,14
In addition, we were limited by the data available in the
NZHIS dataset. For example, we could not adjust for other potential clinical
risk factors such as maternal weight, smoking, and other comorbidities. Health
service information such as time at booking, and maternity carer was not
available. Nor could we link mother’s records to those of the baby. Thus,
factors such as type of carer and baby’s birth weight were unable to be
measured.
However, smoking, obesity, small for gestational age, and a
number of other health status measures are closely correlated with
deprivation8,15–17 and their unobserved
effect may be partially captured by the inclusion of deprivation. The NWH
study5 was able to control for a wider range of
clinical variables. The addition of obstetric risk factors to the model in the
NWH study tended to reduce the odds of Māori having a caesarean section
compared to non-Maori. Therefore, the addition of other such risk factors to the
current analyses may not reduce the disparity between Māori and
non-Māori.
Our results are similar to those found at NWH for ethnic
inequalities in elective caesarean.5 However,
we found that Māori women also had lower adjusted rates of acute CS, which
was not the case at NWH. The differences in these findings may result from the
use of different methods and adjustors or perhaps reflect differences at a
regional compared with a national level.
Internationally, differences in CS have been examined
between various ethnic groups in different countries including the United
States,18-21
Canada,22
Brazil,23 South
Africa,24
Norway,25 and
Australia.26 While there is variation in the
magnitude and direction of ethnic disparities in these studies, in most
studies18–20,22,24,25 ethnic differences
persist after adjusting for clinical and socioeconomic factors, thus suggesting
the influence of non-clinical factors.
Even taking into consideration the limitations of our study,
our findings—which show ethnic differences in CS after adjusting for
socioeconomic position and various clinical factors—raise the possibility
that non-clinical factors may be operating.
Possible non-clinical explanations that may influence ethnic
inequalities in CS have previously been suggested. These include: patient
factors such as maternal request, and patient preferences and
expectations;2,7,27 provider
practice6 and the patient provider
interaction;2 and, differential access to
information and care, and differential
management.2,7
The reasons for ethnic disparities in CS are likely to be
complex and multifactorial, occurring across the continuum of care and
associated with wider determinants of health and inequality. We would argue,
that to address any ‘inequities’ between Māori and
non-Māori, it is important to take a broad perspective to the investigation
of potential explanations.
To focus primarily on patient and Māori
‘cultural’ explanations risks ‘victim
blaming’28 and fails to acknowledge
dominant ‘cultural’ explanations, the role of providers, and
structural influences of the healthcare
system.29 In addition, it does not consider
wider determinants of health and inequality that influence access to care and
individual risk.30,31
Our study shows that disparities in deprivation may
partially contribute to ethnic disparities in CS. However, as a potential risk
factor, ethnic disparities in socioeconomic position alone are limited as they
fail to incorporate factors that lead to the unequal distribution of
socioeconomic resources by ethnicity in the first place. Further research
directly examining potential non-clinical reasons for ethnic disparities in CS
is required within this wider context.
Finally, we note that our study does not determine
appropriate CS rates for Māori and non-Māori, or whether ethnic
disparities in type of delivery contribute to ethnic disparities in birth
outcomes for mothers and babies. Further research is required to assess
this.
Conflict of interest statement: There
are no conflicts of interest.
Author information: Ricci Harris, Public
Health Physician, Te Rōpū Rangahau Hauora a Eru Pōmare,
Department of Public Health, Wellington School of Medicine and Health Sciences,
Otago University, Wellington; Bridget Robson, Director, Te Rōpū
Rangahau Hauora a Eru Pōmare, Department of Public Health, Wellington
School of Medicine and Health Sciences, Otago University, Wellington; Elana
Curtis, Public Health Physician, Senior Lecturer Medical, Te Kupenga Hauora
Māori, Department of Māori Health, University of Auckland, Auckland;
Gordon Purdie, Biostatistician, Department of Public Health, Wellington School
of Medicine and Health Sciences, Otago University, Wellington;
Donna Cormack, Research Fellow, Te Rōpū Rangahau Hauora a Eru
Pōmare, Department of Public Health, Wellington School of Medicine and
Health Sciences, Otago University, Wellington; Papaarangi Reid, Tumuaki, Faculty
of Medical and Health Sciences, University of Auckland, Auckland
Acknowledgements: This study was funded by
a research grant from the Health Research Council of New Zealand and the
Foundation for Research Science and Technology. Dr Ricci Harris was also
provided funding from the John McLeod Scholarship (Australasian Faculty of
Public Health Medicine) to assist with writing this article for
publication.
Correspondence: Dr Ricci Harris, Te
Rōpū Rangahau Hauora a Eru Pōmare, Department of
Public Health, Wellington School of Medicine and Health
Sciences, University of Otago, PO Box 7343,
Wellington. Fax: (04) 385 5924; email Ricci.Harris@otago.ac.nz
References:
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