Joseph, K S. MD, PhD; Dodds, Linda PhD; Allen, Alexander C. MD; Jones, Donna V. MD; Monterrosa, Luis MD; Robinson, Heather MD; Liston, Robert M. MB; Young, David C. MD
The Canadian health care system attempts to embody the egalitarian ideal of distributive justice by providing universal access to essential health services. Several studies1–4 have examined whether Canadians with higher incomes receive preferential access to physician services, specialist physician services, cardiac procedures, and elective surgery. However, the issue of socioeconomic status and receipt of obstetric care services such as labor induction and cesarean delivery has not been studied adequately.
This question is important because in many countries in North and South America, cesarean delivery rates vary widely depending on socioeconomic status. A study from the United States showed that rates of primary cesarean delivery varied from 22.9% among more affluent women to 13.2% among less affluent women, and these differences could not be accounted for by differences in maternal age, parity, complications of pregnancy, and other factors.5 Other studies from the United States have shown substantial differences in labor induction rates as well, with higher rates of such intervention among well-educated women and among those with private insurance.6 Studies from Brazil7–9 also show a wide variation in labor induction and cesarean delivery rates by socioeconomic status, with 1 recent study showing cesarean delivery rates of 22%, 30%, and 50% among women with primary, secondary, and higher education, respectively.9
It is noteworthy that studies examining relationships between socioeconomic status and health services are often limited by the availability of good quality information on socioeconomic status and on the clinical conditions that may influence the need for health interventions. We carried out a study on the relationship between socioeconomic status and receipt of obstetric services using reliable information on socioeconomic status and detailed information on clinical factors.
MATERIALS AND METHODS
All women residents of Nova Scotia, Canada who delivered after a singleton pregnancy between 1988 and 1995 were eligible for inclusion in the study population. Deliveries involving a live birth or a stillbirth at the borderline of viability (ie, gestational age < 20 weeks or a birth weight < 500 g) were excluded to avoid temporal and regional differences in birth registration.10 Information on pregnancy, labor, and delivery events was obtained from the Nova Scotia Atlee Perinatal Database. This population-based database contains detailed information extracted from antenatal and medical charts by trained personnel using standardized forms. An ongoing data quality assurance program, which carries out periodic abstraction studies, and validation studies11 show that the information in the database is reliable.
Information on the income status of study women and their families and other details regarding socioeconomic status were obtained through a linkage between the Nova Scotia Atlee Perinatal Database and the federal income tax (T1) Family Files maintained by the Small Area and Administrative Data Division of Statistics Canada, Ottawa. The linkage was carried out using a combination of deterministic (using social insurance numbers) and probabilistic methods (using nominal information). Tentative links were resolved by a manual examination of records. Of 92,914 women delivering singletons between 1988 and 1995, 76,440 (82.2%) were successfully linked to their income and related information. These confidential linkages and all analyses involving this socioeconomic information were carried out by Small Area and Administrative Data Division personnel in highly secure Small Area and Administrative Data Division offices. Study investigators did not have access to income tax Family File records of individuals at any point. Tabular analyses involving socioeconomic data that resulted in cells with counts less than 15 were suppressed, and all counts were rounded to the nearest 10. Results of regression analyses, which only convey information in the aggregate, were not affected by these confidentiality restrictions.
Income information regarding each woman’s family was obtained for the year in which the delivery occurred. Family income was adjusted for family size by dividing the total family income by the number of members (weighted). The oldest adult received a weight of 1, other members 16 years or older and the first child in a single-parent family received a weight of 0.4, and children younger than 16 years received a weight of 0.3.12 Family income was also adjusted for inflation, with all income being expressed in 1988 Canadian dollars. For analysis purposes, the size- and inflation-adjusted family income was categorized into 5 groups (< $7,500; $7,500 to $13,399; $13,400 to 20,299; $20,300 to $29,299 and ≥ $29,300) based on the original quintile cutoffs created in a pilot study of linkage feasibility. Tax deductible investments (Registered Retirement Savings Plan) made in the year of delivery were also examined as a additional measure that reflects a dimension of socioeconomic status not captured by family income (similar to house and car ownership13). Supplementary analyses were carried out using after-tax income (instead of pretax income), and the correlation between family income in the year before and during which the delivery occurred was also assessed.
Because the study objective was to examine the relationship between socioeconomic status and receipt of obstetric services, we focused on key obstetric services, namely, labor induction and cesarean delivery (ie, the methods of medically indicated early delivery, the cornerstone of modern obstetrics). We also studied the relationship between socioeconomic status and labor induction, overall cesarean delivery, and primary cesarean delivery, with each intervention identified as a separate outcome.
We first examined the association between socioeconomic status and maternal characteristics that could potentially influence rates of labor induction or cesarean delivery. Maternal characteristics assessed included maternal age (< 20, 20–24, 25–29, 30–34, 35–39, and ≥ 40 years), parity (0, 1, 2, ≥ 3), marital status (married, common law, single, other), smoking status at delivery (nonsmoker, 1–9, ≥ 10 cigarettes per day), prepregnancy weight (< 55, 55–59, 60–69, 70–74, and ≥ 75 kg), attendance at prenatal classes (yes or no), and residence in a rural area (yes or no, based on postal code). Relevant obstetric history, including previous cesarean delivery (yes or no), previous low birth weight infant (yes or no), and previous perinatal death (yes or no), was also studied among multiparous women. Other clinical factors, such as specialty of attending physician (obstetrician or other), which is associated with risk status of the pregnancy, and receipt of epidural anesthesia (a risk factor for cesarean delivery but not labor induction) were examined where appropriate. Pregnancy complications studied included hypertensive disorders (yes or no for chronic hypertensive disease; severe pregnancy-induced hypertension; hemolysis, elevated liver enzymes, low platelets syndrome; or eclampsia), gestational diabetes (yes or no), preexisting diabetes mellitus (yes or no), other chronic medical disease (yes or no), placenta previa (yes or no), and placental abruption (yes or no). Other chronic medical disease included cardiovascular disease, renal disease, gastrointestinal disorders, major psychiatric illness, neurologic disorders, endocrine disease, neoplastic disease, and blood dyscrasias.
Statistical comparisons of proportions between socioeconomic status categories (eg, lowest compared with highest) were carried out using χ2 tests. The relationship between socioeconomic status and obstetric intervention was then modeled using logistic regression, and the odds ratios obtained were converted to rate ratios (because all outcomes were relatively common).14 Variance estimates were adjusted (Proc Genmod, SAS 8.2, SAS Institute Inc., Cary, NC) to account for the potential nonindependence of observations,15 because some women had more than 1 delivery during the study period. The database linkage was approved by the Policy Committee at Statistics Canada and the study was approved by the Dalhousie University Research Ethics Board.
The proportion of women with advanced maternal age was substantially higher among the higher compared with the lower socioeconomic categories; 17.7% of women with a family income of $29,300 or more were aged 35 years or older compared with 4.4% of women with a family income less than $7,500 (P < .001, Table 1). A similar socioeconomic differential was also evident when women were categorized by family contribution to an investment (Registered Retirement Savings Plan). The socioeconomic gradient with regard to nulliparous older women was even more pronounced, with 1.2% in the lowest income group and 10.8% in the highest income group being first-time mothers at an age of 35 years or older (P < .001). Differences in other maternal characteristics were also observed across socioeconomic categories, with women in lower socioeconomic groups having lower rates of marriage and prenatal class attendance and higher rates of residence in rural areas, smoking, and prepregnancy weight of 75 kg or more.
Figure 1 shows the distribution of various maternal characteristics in the early and later periods of the study. Although there were substantial temporal trends in older maternal age, prepregnancy weight, smoking, and other factors, the relative distribution of each factor by socioeconomic status remained essentially unchanged. Women in higher socioeconomic groups had higher rates of epidural anesthesia, whereas socioeconomic differences in the proportion of women who had a obstetrician as their attending physician were less evident (Fig. 1).
Among multiparous women with a family income of $29,300 or more, 19.2% had a previous cesarean delivery compared with 17.4% of women with a family income less than $7,500 (P = .01, Table 2). Multiparous women in the lower income categories had higher rates of previous low birth weight and previous perinatal death. This association was also present when the women were categorized by contribution status to an investment (Registered Retirement Savings Plan). There were small differences between socioeconomic groups with regard to the frequency of various pregnancy complications, but no clear patterns were evident, because rates were low (Table 2).
Rates of labor induction increased substantially between 1988–1990 and 1993–1995 (Fig. 1). However, there was little difference in crude labor induction rates across socioeconomic status categories (17.7% in the lowest income group compared with 18.1% in the highest income group, Table 3). On the other hand, crude rates of overall cesarean delivery and primary cesarean delivery were significantly lower among lower income groups compared with higher income groups. Crude rate ratios for overall cesarean delivery were 0.84 (95% CI 0.80–0.89) times lower among women in the lowest income group as compared with women in the highest income category. Similarly, the crude rate ratio for primary cesarean delivery was 0.73 (95% CI 0.69–0.78).
Adjustment for potential confounders showed that compared with women in the highest income category, women in the lowest income category were 1.09 (95% CI 1.00–1.19, P = .05) times as likely to have had their labor induced (Table 3). Adjustment also revealed that women in the lower income categories had higher rates of cesarean delivery (when relevant risk factors were controlled) compared with women in the highest income group. The adjusted rate ratio comparing the lowest with highest income group was 1.12 (95% CI 1.03–1.23, P = .01) for overall cesarean delivery and 1.09 (95% CI 0.98–1.21, P = .12) for primary cesarean delivery (Table 3). Noncontribution to an investment plan (relative to contribution to an investment plan and adjusted for income category and other factors) was also associated with a 12% (95% CI 6–18%) higher rate of overall cesarean delivery and a 13% (95% CI 6–21%) higher rate of primary cesarean delivery but not with differences in labor induction (adjusted rate ratio 0.98, 95% CI 0.94–1.03, Table 3).
Results from partial, sequential models showed how the crude relationship between socioeconomic status and obstetric intervention was altered by regression adjustment for potential confounders (Table 4). The crude rate ratio for labor induction was most influenced by adjustment for maternal characteristics (ie, age, parity, prepregnancy weight, smoking, marital status, rural residence, and prenatal class attendance) and also by other factors (ie, specialty of attending physician and time period). Similarly, adjustment for maternal characteristics substantially changed the rate ratio in the overall cesarean and primary cesarean delivery models. The relationship between family income and overall cesarean delivery was also affected by control for other factors such as epidural anesthesia, specialty of attending physician, and time period. Figure 2 illustrates how control of confounding by maternal characteristics (age and prepregnancy weight) changes the pattern of cesarean delivery rates by family income. Differences in overall rates of cesarean delivery among all women (P value for crude comparison between lowest compared with highest family income groups = .01, Table 3), were not observed in the subpopulation of women between 20 and 34 years and 50 and 69 kg (P value comparing lowest with highest family income groups = .72).
Table 5 shows the results of regression analysis with labor induction or cesarean delivery as the outcome. Adjusted rate ratios for family income showed that women in the 3 lower family income groups had higher rates of obstetric intervention compared with women in the highest family income category. Similarly, noncontribution to an investment plan was associated with a significantly higher rate of labor induction or cesarean delivery. Maternal characteristics, obstetric history, pregnancy complications, and other clinical factors were all closely associated with obstetric intervention.
Supplementary analyses carried out to assess the effect of using alternative indices for quantifying socioeconomic status yielded essentially the same results. For instance, analyses using after tax income instead of pretax income provided similar estimates of effect. There was substantial correlation between income in the year of delivery and income in the previous year. Women excluded from the analysis (unlinked records) had a labor induction rate of 15.6% (compared with 17.4% among women whose records were linked) and a cesarean delivery rate of 17.1% (compared with 19.2% among the linked).
Our study shows that women in higher socioeconomic strata were not treated preferentially under the Canadian health care system with regard to labor induction and cesarean delivery. Adjusted rates of labor induction and overall cesarean delivery were significantly higher among lower income groups relative to higher income groups. Differences were also observed in contrasts involving other indices of socioeconomic status, namely, contribution to an investment (Registered Retirement Savings Plan) plan, with noncontribution associated with substantially higher adjusted rates of overall cesarean and primary cesarean delivery.
It is encouraging although perhaps not surprising to see higher adjusted rates of obstetric intervention among lower socioeconomic status women in the Canadian setting. The Medical Care (Medicare) Act enacted in 196816 instituted medical insurance for all Canadian citizens. Currently, the Canada Health Act17 ensures that all provinces and territories uphold the 5 principles of universality, comprehensiveness, portability, public administration, and accessibility to receive federal funding for health care. Under this system, physicians and hospitals providing obstetric services have no financial or other incentive to offer different services to affluent and less affluent women. Thus, it seems reasonable to generalize our findings to all provinces and territories of Canada, even though our study was restricted to Nova Scotia.
It is also encouraging to note that obstetric intervention is being offered appropriately. The risk factors for labor induction or cesarean delivery identified in our regression models are fully consistent with clinical expectation, with adjusted rate ratios (and especially odds ratios) being strongly associated with pregnancy complications such as placenta previa, hypertensive disorders, diabetes mellitus, and placental abruption (Table 5). Previous cesarean delivery and previous perinatal death were also closely associated with higher rates of subsequent obstetric intervention. Higher-risk categories of maternal age, parity, and prepregnancy weight showed expected gradations in the risk of obstetric intervention. The higher rates of obstetric intervention among lower socioeconomic groups are a reflection of the appropriateness of obstetric care, given that such women are typically less healthy and more likely to require such services.18 It is generally recognized that socioeconomic status is associated with a host of lifestyle, behavioral, and related factors that cannot all be readily quantified.19
The observed behavioral correlates of socioeconomic status among pregnant women were not unexpected given similar associations in previous studies.20–22 Such relationships between socioeconomic status and health behaviors and other clinical factors underscore the need for comprehensive control of such confounders in studies of socioeconomic differentials in health service delivery. In our study, women in higher socioeconomic groups had higher crude rates of overall and primary cesarean delivery, and this relationship was only reversed after adjustment for maternal characteristics, previous obstetric history, pregnancy complications, and other factors. Other studies based on vital records, which have shown lower rates of cesarean delivery among women receiving welfare and the like,23 may have been limited by an inability to control for such a diverse set of relevant confounders. On the other hand, access to health services in Canada is known to be less than optimal in remote communities and among aboriginal subpopulations.24,25
It is interesting to note the distribution and temporal trends in rates of older maternal age among the different socioeconomic groups. This risk factor for obstetric intervention and adverse perinatal outcomes (such as preterm birth, small for gestational age, perinatal death, and serious neonatal morbidity26–28) is substantially more frequent among the higher socioeconomic groups. The relationship between socioeconomic status and older nulliparous mothers is even more striking (Fig. 1) and of significance, given that both older maternal age and nulliparity are independent risk factors for obstetric intervention and adverse perinatal outcomes.
The limitations of our study pertain to the noninclusion of home births and a lack of information on educational status, a factor associated with perinatal outcomes.29,30 However, home births constituted about 0.5% of births in Nova Scotia during the study period, and we did adjust for various correlates of education including marital status, attendance at prenatal classes, smoking status, prepregnancy weight, parity, and family income. Nevertheless, our indices of socioeconomic status (family income and tax deductible investment) focus more on economic and less on social aspects. Other limitations of our study include an inability to link all records and some missing values for risk factors that were included in regression models. Nevertheless, our final regression models were based on 55,520 deliveries.
In conclusion, our study shows that socioeconomic groups differ substantially with regard to risk factors that determine the need for obstetric intervention. However, labor induction, overall cesarean delivery, primary cesarean delivery, and labor induction and cesarean delivery rates were not higher among more affluent women when all relevant risk factors are controlled during analysis. Thus, the Canadian health care system, which is expected to provide essential health care services to all its citizens, seems to serve its obstetric population on the basis of medical need and without regard to socioeconomic status.
1. Dunlop S, Coyte PC, McIsaac W. Socio-economic status and the utilisation of physicians’ services: results from the Canadian National Population Health Survey. Soc Sci Med 2000;51:123–33.
2. Alter DA, Naylor CD, Austin P, Tu JV. Effects of socioeconomic status on access to invasive cardiac procedures and on mortality after acute myocardial infarction. N Engl J Med 1999;341:1359–67.
3. Finkelstein MM. Do factors other than need determine utilization of physicians’ services in Ontario? CMAJ 2001;165:565–70.
4. Shortt SE, Shaw RA. Equity in Canadian health care: does socioeconomic status affect waiting times for elective surgery? CMAJ 2003;168:413–6.
5. Gould JB, Davey B, Stafford RS. Socioeconomic differences in rates of cesarean section. N Engl J Med 1989;321:233–9.
6. Coonrod DV, Bay RC, Kishi GY. The epidemiology of labor induction: Arizona, 1997. Am J Obstet Gynecol 2000;182:1355–62.
7. Belizan JM, Althabe F, Barros FC, Alexander S. Rates and implications of caesarean sections in Latin America: ecological study. BMJ 1999;319:1397–400.
8. Barros FC, Vaughan JP, Victora CG, Huttly SR. Epidemic of caesarean sections in Brazil. Lancet 1991;338:167–9.
9. Fabri RH, Murta EF. Socioeconomic factors and cesarean section rates. Int J Gynaecol Obstet 2002;76:87–8.
10. Joseph KS, Kramer MS. Recent trends in Canadian infant mortality rates: effect of changes in the registration of live newborns weighing less than 500 g. CMAJ 1996;155:1047–52.
11. Fair M, Cyr M, Allen AC, Wen SW, Guyon G, Macdonald RC. Validation study for a record linkage of births and infant deaths in Canada. Catalogue No. 84F0013XIE. Ottawa (Canada): Statistics Canada; 1999.
12. Low income cutoffs from 1991 to 2000 and low income measures from 1990 to 1999. Catalogue No. 75F0002MIE-01007. Income Statistics Division. Ottawa (Canada): Statistics Canada; 2001.
13. Smith GD, Shipley MJ, Rose G. Magnitude and causes of socioeconomic differentials in mortality: further evidence from the Whitehall Study. J Epidemiol Community Health 1990;44:265–70.
14. Zhang J, Yu KF. What’s the relative risk? A method of correcting the odds ratio in cohort studies of common outcomes. JAMA 1998;280:1690–1.
15. Zeger SL, Liang KY. Longitudinal data analysis for discrete and continuous outcomes. Biometrics 1986;42:121–30.
16. Canada Medical Care Act, 4 Eliz 2, c64 (1966).
17. Canada. Canada Health Act. c. 6, s. 1 (1984).
18. Kramer MS, Goulet L, Lydon J, Seguin L, McNamara H, Dassa C, et al. Socio-economic disparities in preterm birth: causal pathways and mechanisms. Paediatr Perinat Epidemiol. 2001;15:104–23.
19. Joseph KS, Kramer MS. Review of the evidence on fetal and early childhood antecedents of adult chronic disease. Epidemiol Rev 1996;18:158–74.
20. Cnattingius S, Lambe M. Trends in smoking and overweight during pregnancy: prevalence, risks of pregnancy complications, and adverse pregnancy outcomes. Semin Perinatol 2002;26:286–95.
21. Phares TM, Morrow B, Lansky A, Barfield WD, Prince CB, Marchi KS, et al. Surveillance for disparities in maternal health-related behaviors–selected states, Pregnancy Risk Assessment Monitoring System (PRAMS), 2000-2001. MMWR Surveill Summ 2004;53:1–13.
22. Pomerleau J, Pederson LL, Ostbye T, Speechley M, Speechley KN. Health behaviours and socio-economic status in Ontario, Canada. Eur J Epidemiol 1997;13:613–22.
23. Johnson D, Jin Y, Truman C. Influence of aboriginal and socioeconomic status on birth outcome and maternal morbidity. J Obstet Gynaecol Can 2002;24:633–40.
24. Tonelli M, Hemmelgarn B, Manns B, Pylypchuk G, Bohm C, Yeates K, et al. Death and renal transplantation among Aboriginal people undergoing dialysis. CMAJ 2004;171:577–82.
25. Booth GL, Hux JE, Fang J, Chan BT. Time trends and geographic disparities in acute complications of diabetes in Ontario, Canada. Diabetes Care 2005;28:1045–50.
26. Salihu HM, Shumpert MN, Slay M, Kirby RS, Alexander GR. Childbearing beyond maternal age 50 and fetal outcomes in the United States. Obstet Gynecol 2003;102:1006–14.
27. Jacobsson B, Ladfors L, Milsom I. Advanced maternal age and adverse perinatal outcome. Obstet Gynecol 2004;104:727–33.
28. Joseph KS, Allen AC, Dodds L, Turner LA, Scott S, Liston R. The perinatal effects of delayed childbearing. Obstet Gynecol 2005;105:1410–8.
29. Chen J, Fair M, Wilkins R, Cyr M. Maternal education and fetal and infant mortality in Quebec. Fetal and Infant Mortality Study Group of the Canadian Perinatal Surveillance System. Health Rep 1998;10:53–64.
30. Health Canada. Canadian Perinatal Health Report 2003. Ottawa (Canada): Canadian Perinatal Surveillance System; 2003.