A population-level analysis of birth weight indices in peel region, Ontario: the impact of ethnic diversity.
Ramuscak, Nancy L. ; Jiang, Depeng ; Dooling, Kathleen L. 等
The rate of low birth weight (LBW), defined as a birth weight of
less than 2500 grams, is a common measure of a population's
reproductive health. The two components of LBW are: preterm birth and
intrauterine growth restriction (IUGR). Infants born at a birth weight
below the sex-specific 10th percentile for their gestational age are
termed small-for-gestational-age (SGA) and may have experienced IUGR.
The risk factors associated with preterm birth and being SGA are
different (1) and therefore each component must be examined separately
to understand the causes of LBW in a population.
The ethnic origin and immigrant status of mothers impacts rates of
preterm birth and SGA. (2-7) At a population level, Caribbean women in
England and Wales had a higher singleton preterm birth rate compared to
white British women (9.7% vs. 6.1%). (3) Infants of South Asian or East
Asian mothers have lower birth weights than those of Canadian-born
mothers and consequently have higher rates of SGA. (2,5) These studies
do not address whether these differences are attributable to
ethnic-specific risk for certain conditions during pregnancy (e.g.,
pre-eclampsia), natural differences in stature and body composition, or
other reasons.
The Regional Municipality of Peel, Ontario (which includes the
municipalities of Caledon, Brampton and Mississauga) has an elevated
rate of LBW compared to that of Ontario (6.8% vs. 6.0%), prompting
concern among public health practitioners. The population of Peel is
ethnically diverse, with almost half of residents being immigrants to
Canada. The purpose of this paper is to examine whether the elevated
rate of LBW in the Region of Peel, Ontario can be attributed to the high
proportion of immigrants in the population. In addition, we examined how
the infant birth weight distribution in Peel differs by maternal region
of birth.
METHODS
Provincial birth registration data were used to examine LBW,
preterm birth and SGA rates. Data were obtained for the five-year period
of 2002 through 2006, which were the most recent years of data for which
maternal region of birth was available. Infants born to mothers who
reside in Peel were compared to all infants in the province. Preterm
birth is defined as a gestational age less than 37 completed weeks.
Singleton infants were defined as SGA if they had a birth weight below
the sex-specific 10th percentile according to the Canadian standard
developed by Kramer and colleagues in 2001 (valid for gestational ages
between 22 and 43 weeks only). (8) Multiple births were excluded from
the SGA analysis.
Provincial birth registration records do not capture information
regarding the ethnic origin of an infant. Therefore, the self-reported
country of birth of the mother was used as a proxy measure for her
ethnic origin. The country of birth was categorized into geographical
regions based on categories utilized by Statistics Canada. (9) The
categories examined in this study were: Canada, South Asia, East Asia
(includes both Eastern and Southeast Asian countries), Caribbean,
African (excludes Northern Africa), other countries and unknown. Data
regarding the mother's country of birth was missing for 0.1% of
records, and therefore these records were excluded from the analysis.
Mothers born in Canada were defined as "non-immigrants",
whereas mothers born outside of Canada were defined as
"immigrants". Although data are available regarding the
country of birth for both parents, the maternal region of birth was
selected for analysis as the data regarding the maternal and paternal
country of birth were found to be highly correlated (80.5% agreement,
kappa = 0.67).
LBW, preterm birth and SGA rates for infants in Peel and all of
Ontario were calculated and compared using SPSS Version 19.0 (IBM
Company Inc, USA) and EpiInfo 6.04d (Centers for Disease Control and
Prevention, Atlanta, GA). All p-values were two-sided, and significance
was set at a value of 0.05.
To examine whether the differences in rates observed were the
result of a shift in the underlying birth weight distribution of infants
of immigrant mothers, birth weight for gestational age curves which were
specific for infant sex and maternal region of birth were developed.
These curves were developed for all singleton births in Peel from 2002
to 2006 using lmsChartMaker Light (Medical Research Council, UK). Weight
percentile curves were derived using the lambda-mu-sigma (LMS) method.
(10) The L (skewness), M (median) and S (dispersion) parameters were
estimated, smoothed using the penalized likelihood method, and fitted as
cubic splines by nonlinear regression. The penalized likelihood, with
the roughness penalties incorporated with the likelihood, makes the
smoothing of the three curves become an integral part of the likelihood
maximization. Maximizing the penalized likelihood strikes a balance
between fidelity to the data and smoothness of the L, M and S curves.
Data for 50th percentile (median) among male and female infants are
presented here. Data corresponding to other percentile values can be
obtained from the authors, upon request. Using the smoothed curve data,
we conducted quantile regression to examine whether the percentiles of
birth weight among term infants (gestational age between 37 and 42
weeks) differed by the region of birth of the mother. Quantile
regression models the relation between a set of predictor variables and
specific percentiles (or quantiles) of the response variable. It
specifies changes in the quantiles of the response. For example, a
median regression of infant birth weight on mothers'
characteristics (e.g., region of birth) specifies the changes in the
median birth weight as a function of the predictors.
RESULTS
Table 1 shows the maternal and infant characteristics of the live
births in Peel (N=74,836) and Ontario (N=659,802) for the time period
2002 through 2006. Live births in Peel accounted for slightly more than
10% of all live births in the province in each year. The average age of
mothers was similar between Peel (30.2 years) and Ontario (29.9 years).
In Peel, 38% of mothers were born in Canada (non-immigrants), in
comparison to 64% of all Ontario mothers who were born in Canada. Among
mothers in Peel, 26% were born in South Asia (42% of all mothers born
outside of Canada).
The LBW rate in Peel was significantly higher than that in Ontario
(6.8% vs. 6.0%, p<0.001). There was no difference in the preterm
birth rate comparing Peel to Ontario, whereas the SGA rate in Peel was
significantly higher than that in Ontario (10.8% vs. 8.7%, p<0.001)
(Table 1).
South Asian, Caribbean and African mothers had significantly higher
rates of LBW than Canadian-born mothers (Table 2). Caribbean mothers had
the highest rate of preterm births (10.7%). South Asian and East Asian
mothers had significantly lower preterm birth rates compared to
Canadian-born mothers. Immigrant mothers had higher SGA rates than
non-immigrant mothers, regardless of where they were born, with South
Asian mothers having the highest rate.
Table 3 includes the birth weight values associated with the 50th
percentile (median) for male and female Peel infants at each gestational
age (between 31 to 42 weeks) for each maternal region of birth. Infants
born to immigrant mothers had lower median birth weights than those born
to non-immigrant mothers across most gestational ages. At the earliest
gestational ages (before 34 weeks), this pattern is not consistent among
infants of East Asian-born and Caribbean-born mothers. However, this may
be due to the small number of infants in these categories leading to
variability in the median values or to fetal fat accumulation during the
last weeks of pregnancy.
Infants of all immigrant mothers born at term have lower median
birth weight compared to infants of non-immigrant mothers (Table 4). The
largest difference in birth weight at term was observed between infants
born to South Asian-born and those born to Canadian-born mothers. For
example, infants of South Asian-born mothers had a significantly lower
median birth weight at term compared to infants of Canadian-born mothers
(220-gram difference among male infants and 189-gram difference among
female infants).
DISCUSSION
Our findings use population data to show that the elevated rate of
LBW in Peel is a result of higher rates of SGA and not due to
differences in rates of preterm birth. Moreover, infants born to
immigrant mothers have lower birth weight at all gestations and
consequently higher SGA rates than infants born to non-immigrant
mothers. The median birth weight for infants of immigrant mothers was
lower than that for infants of non-immigrant mothers, showing that the
underlying birth weight distribution for infants of immigrant mothers is
shifted towards lower values compared to that for infants of
Canadian-born mothers. The high proportion of immigrants of childbearing
age in Peel and the shifted birth weight distribution results in the
elevated LBW rate observed for Peel.
Compared to the province of Ontario, Peel's population has a
high proportion of immigrants, which is reflected among the cohort of
infants born each year. The majority of births in Peel occur to
immigrant mothers, with a large proportion occurring among immigrant
mothers from South Asia, East Asia and the Caribbean. As evidenced here,
for geographic areas with a high proportion of immigrants, the
difference in birth weight distribution will have an important impact on
common reproductive health measures such as LBW and SGA rates.
Given that newborns of immigrant mothers have a lighter birth
weight distribution across all gestational ages, using the Canadian
fetal growth standard developed by Kramer and colleagues (2001) to
assess the appropriateness of the birth weight of an infant born to an
immigrant mother will potentially result in misclassification. Ray and
colleagues reported that 111 per 1,000 male and 88 per 1,000 female
newborns of South Asian origin may be misclassified as SGA using the
Canadian standard compared to the ethnic specific standard derived based
on hospital data. (5) Misclassifying infants of immigrant mothers as
small-for-gestational-age when they may have an appropriate birth weight
for their gestational age and ethnicity may lead to unnecessary
additional health care costs and parental stress. Ethnic-specific fetal
growth standards developed using population birth data from British
Columbia were shown to be more congruent with observed perinatal
mortality than the single growth standard (i.e., not ethnic-specific),
leading the authors to conclude that the differences in birth weight by
ethnic group represent a physiologic rather than a pathologic process.
(2)
At term, infants of immigrant mothers had lower median birth weight
values compared to infants of non-immigrant mothers. At a population
level, these observed differences are significant and it follows that a
higher proportion of infants born to immigrant mothers would be expected
to fall below 2500 grams and, therefore, meet the criteria for LBW, as
seen in Peel.
This paper does not provide guidance regarding the clinical
assessment of individual infants, but rather provides a description of
trends in birth weight indices at the population level in Peel. Clinical
decisions and follow-up regarding LBW infants must be determined by
health care providers based on factors relevant to each individual
infant.
There are limitations to the analysis presented in this paper that
may have an impact on the applicability of the results. It was not
possible to analyze ethnic origin of the infant's mother and
therefore the country of birth of the mother was used as a proxy. The
regions included for analysis are geographically large and diverse and
therefore will include mothers with a range of cultural and ethnic
backgrounds (including the Canadian-born mothers). (11) Data from the
2007/2008 Canadian Community Health Survey show that 10% of
non-immigrants in Peel self-identified as South Asian and 3% as East or
Southeast Asian. The inclusion of mothers of South and East Asian ethnic
origins in the non-immigrant category would lower the birth weight
distribution observed in this category. Therefore our findings may be
considered as a conservative estimate of difference in birth weight
distributions between ethnicities.
Due to the limited number of variables available within the
provincial birth registration dataset, we were unable to adjust for
known risk factors for fetal growth restriction or preterm birth (e.g.,
smoking during pregnancy, diabetes and hypertension, the length of time
since arrival in Canada, mental health issues, inadequate prenatal care)
which may have an important impact on perinatal outcomes. (6,7,12)
Census data show that Peel has a young, highly educated population with
a median income similar to the rest of the province of Ontario. Five
percent of Peel mothers in 2008 reported smoking during pregnancy. The
prevalence of diabetes and hypertension among Ontario mothers was 6% and
5% in 2006/2007, respectively. (13) It is difficult to know how much of
an effect these unmeasured confounders might have on our findings
because recent immigrants tend to have fewer risk factors than
non-immigrants or long-term immigrants.
Last, there are a number of widely recognized data quality issues
with the Ontario birth registration data, including under-registration
of births, missing birth registration data for infants who have died,
and inaccurate gestational age recorded for births between 1991 and
1998. (8,14-17) Birth registration data are collected both from
parents/guardians and the health care provider present at the birth.
Parent-reported data are subject to recall bias and may not align with
data reported by the health care provider. For the time period examined
in this paper, information about the infant was obtained from the data
provided by the health care provider (e.g., gestational age). The
mother's country of birth is collected from the parent-report data
and cannot be verified. In addition, examination of the 2006 data
revealed that the birth weight values unnaturally cluster at values that
correspond to a converted number of even pounds and ounces. Although
this conversion leads to imprecision of birth weight, there is no reason
to believe that the direction of the error would lead to significant
bias.
The observed difference in LBW rates between Peel and Ontario can
be explained by the higher proportion of infants born to immigrant
mothers in Peel. Rates of preterm birth were similar between Peel and
Ontario, whereas the SGA rate was significantly higher in Peel and
highest among infants born to immigrant mothers. This higher SGA rate
reflects the shift towards lower birth weights across gestation among
infants of immigrant mothers. Further analysis may be able to determine
whether the lower birth weight among infants of immigrant mothers leads
to higher rates of mortality or morbidity compared to infants of
non-immigrant mothers.
Financial support provided by: Health Services, Regional
Municipality of Peel
Conflict of Interest: None to declare.
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(14.) Woodward GL, Bienefeld MK, Ardal S. Under-reporting of live
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(15.) Public Health Agency of Canada. Canadian Perintal Health
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Received: March 9, 2012
Accepted: July 18, 2012
Nancy L. Ramuscak, MSc, [1] Depeng Jiang, PhD, [2] Kathleen L.
Dooling, MD, [1] David L. Mowat, MBChB [1]
Author Affiliations
[1.] Health Services, Regional Municipality of Peel, Mississauga,
ON
[2.] Department of Community Health Science, Faculty of Medicine,
University of Manitoba, Winnipeg, MB
Correspondence: Nancy Ramuscak, Health Services, Regional
Municipality of Peel, 7120 Hurontario Street, P.O. Box 667, RPO
Streetsville, Mississauga, ON L5M 2C2, Tel: 905-791-7800, ext. 2423,
Fax: 905-564-2683, E-mail: nancy.ramuscak@peelregion.ca
Table 1. Maternal and Infant Characteristics
Peel Ontario
n % of n % of
Live Live
Births Births
2002-2006 2002-2006
Year of birth
2002 13,488 18.0 128,202 19.4
2003 14,438 19.3 130,603 19.8
2004 15,190 20.3 132,221 20.0
2005 15,738 21.0 133,485 20.2
2006 15,982 21.4 135,291 20.5
2002-2006 74,836 100.0 659,802 100.0
Maternal age (years)
Mean 30.2 29.9
Median 30.0 30.0
Range 12-52 12-55
Type of birth
Singleton 72,419 96.8 638,758 96.8
Multiple 2417 3.2 21,044 3.2
Maternal region of birth
Canada 28,495 38.1 420,529 63.7
South Asia 19,661 26.3 56,223 8.5
East Asia 6838 9.1 51,216 7.8
Caribbean 3988 5.3 16,636 2.5
Africa 2122 2.8 16,713 2.5
Other 13,663 18.3 97,749 14.8
Low birth 5126 6.8 39,791 6.0
weight
Preterm birth 5608 7.5 49,442 7.5
SGA * 7826 10.8 55,703 8.7
* SGA rate calculated for singleton births between 22
and 43 weeks gestation only.
Table 2. Birth Outcomes by Maternal Region of Birth, Peel Region,
2002-2006
Maternal Low Birth Weight Preterm Birth Small-for
Region -gestational
of Birth -age
n % n % n %
Canada 1790 6.3 2256 7.9 2128 7.8
South 1594 8.1 1385 7.0 2972 15.5
Asia ([dagger]) ([dagger]) ([dagger])
East 385 5.6 450 6.6 761 11.3
Asia ([double ([dagger]) ([dagger])
dagger])
Caribbean 388 9.7 425 10.7 516 13.4
([dagger]) ([dagger]) ([dagger])
Africa 168 7.9 179 8.4 232 11.3
([dagger]) ([dagger])
Other 789 5.8 898 6.6 1208 9.11
([double ([dagger]) ([dagger])
dagger])
Total 5126 6.8 5608 7.5 7826 10.8
* SGA rate calculated for singleton births between 22 and 43 weeks
gestation only.
([dagger]) Difference in proportions significant at p<0.01 level
(compared to Canada reference).
([double dagger]) Difference in proportions significant at p<0.05
level (compared to Canada reference).
Table 3. Birth Weight at the 50th Percentile by Sex,
Gestational Age and Maternal Region of Birth
Infant Gestational Maternal Region of Birth
Sex Age
(weeks)
Canada South East
(grams) Asia Asia
(grams) (grams)
Female 31 1495.3 1489.8 1667.2
32 1766.1 1718.5 1841.7
33 2035.3 1945.7 2032.5
34 2298.5 2177.1 2240.1
35 2553.5 2415.2 2459.6
36 2797.6 2651.0 2688.1
37 3022.9 2873.9 2917.4
38 3219.9 3059.8 3120.6
39 3382.2 3209.3 3256.2
40 3515.3 3334.1 3382.2
41 3632.0 3426.3 3500.3
42 3743.0 3511.1 3604.2
Male 31 1697.2 1520.7 1582.3
32 1937.5 1758.3 1814.4
33 2174.4 1997.3 2049.3
34 2406.9 2234.3 2290.4
35 2644.5 2471.0 2540.8
36 2894.2 2713.2 2792.7
37 3143.8 2956.3 3026.3
38 3358.7 3171.6 3221.9
39 3520.1 3328.8 3364.4
40 3660.9 3455.6 3481.3
41 3783.5 3580.1 3602.5
42 3899.7 3703.0 3720.9
Infant Gestational
Sex Age
(weeks)
Caribbean Africa Other
(grams) (grams) (grams)
Female 31 1551.8 1767.8 1554.9
32 1755.4 1891.9 1795.8
33 1968.7 2024.5 2037.0
34 2205.9 2199.3 2278.8
35 2463.0 2428.9 2523.6
36 2720.2 2714.5 2763.7
37 2944.1 2993.5 2988.5
38 3118.1 3187.6 3181.1
39 3262.5 3314.4 3325.0
40 3395.4 3403.2 3466.9
41 3501.1 3484.1 3602.3
42 3598.8 3572.6 3729.5
Male 31 1580.3 1468.3 1576.2
32 1800.8 1742.4 1818.4
33 2017.1 2021.3 2062.9
34 2244.4 2296.4 2316.2
35 2487.1 2561.8 2580.5
36 2742.4 2809.0 2844.8
37 2994.5 3039.7 3094.6
38 3201.2 3237.8 3303.0
39 3369.3 3390.6 3469.3
40 3498.5 3535.8 3617.2
41 3620.1 3677.7 3732.9
42 3735.4 3811.9 3832.5
Table 4. Median Birth Weight for Male and Female Term *
Infants by Maternal Region of Birth
Maternal Males
Region
of Birth
50th Percentile Difference
Birth Weight at 50th
(grams) Percentile
(95% CI)
Canada 3555 Reference
South Asia 3335 -220g ([dagger])
(-241g, -199g)
East Asia 3345 -210g ([dagger])
(-230g, -190g)
Caribbean 3373 -182g ([dagger])
(-206g, -158g)
Africa 3430 -125g ([dagger])
(-165g, -85g)
Other 3514 -41g ([dagger])
(-59g, -23g)
Maternal Females
Region
of Birth
50th Percentile Difference at
Birth Weight 50th Percentile
(grams) (95% CI)
Canada 3415 Reference
South Asia 3226 -189g ([dagger])
(-207g, -171g)
East Asia 3260 -155g ([dagger])
(-175g, -135g)
Caribbean 3271 -143g ([dagger])
(-171g, -116g)
Africa 3316 -99g ([dagger])
(-135g, -61g)
Other 3370 -45g ([dagger])
(-63g, -27g)
* Term infants are between 37 and 42 completed weeks of
gestation at birth.
([dagger]) Difference significant at p<0.001 level.
