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ORIGINAL RESEARCH |
From the Perinatal Epidemiology Research Unit, Departments of Obstetrics and Gynecology and Pediatrics and the Division of Maternal/Fetal Medicine, Department of Obstetrics and Gynecology, Dalhousie University Faculty of Medicine, Halifax, Nova Scotia, Canada.
Address reprint requests to: K. S. Joseph, MD, PhD, IWK Health Centre, Division of Neonatal Pediatrics, 5980 University Avenue, Halifax, NS B3H 4N1, Canada; E-mail: kjoseph{at}is.dal.ca.
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ABSTRACT |
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METHODS: We studied all deliveries in Nova Scotia, Canada, between 1988 and 2000 after excluding women who had a previous cesarean delivery (n=127,564). Logistic regression was used to study the effect of changes in maternal characteristics and obstetric practice on primary cesarean delivery rates. The effect of changes in midpelvic forceps delivery was examined through ecologic Poisson regression.
RESULTS: Primary cesarean delivery rates increased from 13.4% of deliveries in 1988 to 17.5% in 2000. This was due to increases in cesarean deliveries for dystocia (14% increase), breech (24% increase), suspected fetal distress (21% increase), hypertension (47% increase), and miscellaneous indications (73% increase). Adjustment for maternal characteristics reduced the temporal increase in primary cesarean delivery rates between 1988–1991 and 1998–2000 from 21% (95% confidence interval [CI] 16%, 25%) to 2% (95% CI -2%, 7%). Additional adjustment for obstetric practice factors further reduced period effects. Midpelvic forceps delivery was significantly and negatively associated with primary cesarean delivery (P = .001).
CONCLUSION: Recent increases in primary cesarean delivery rates are a consequence of changes in maternal characteristics. Obstetric practice, which has altered due to changes in maternal characteristics and concerns related to fetal and maternal safety, has also contributed to increases in primary cesarean delivery.
Over the last few years, cesarean delivery rates have increased in several industrialized countries.1–4 In Canada, the cesarean delivery rate increased from 17.8% in 1994 to 19.1% in 1997, and in the United States, rates have increased for 4 successive years, from 20.7% in 1996 to 22.9% in 2000.1,2 In England and Wales, the cesarean delivery rate increased from 16% in 1995 to 19% in 1999 and 21.5% in 2000.3 Increases in primary cesarean delivery rates mirror the rising overall rates of cesarean delivery. In Canada, the rate of primary cesarean delivery increased from 12.6% in 1994 to 13.8% in 1997.1 Similarly, in the United States, the primary cesarean delivery rate increased from 14.6% in 1996 to 16.1% in 2000.2 This escalation in cesarean delivery rates has occurred against a background of prescriptive guidelines and targets.5,6
Cesarean delivery rates in the United States increased four-fold between the mid-1960s and the late 1980s.7 Explanations for this increase include changes in maternal characteristics (increases in older maternal age, reduced parity), obstetric practice (increasing use of electronic fetal monitoring, cesarean delivery for breech presentation, epidural anesthesia, and reduced use of midpelvic forceps), and social factors (malpractice litigation and socioeconomic factors).7,8 The reasons for the more recent increases in cesarean delivery rates since the mid-1990s are less well understood. Although concerns about maternal safety have tempered the enthusiasm for vaginal birth after cesarean delivery,9 the reasons for the increase in primary cesarean delivery rates have not been adequately studied. However, several recent studies4,10,11 have identified maternal factors, such as age, parity, and prepregnancy weight, as the explanatory factors underlying recent trends in cesarean delivery.
Primary cesarean delivery rates have been increasing in the Canadian province of Nova Scotia in parallel with recent increases in such deliveries in the rest of Canada and the United States. We carried out a study to examine changes in primary cesarean delivery rates between 1988 and 2000 to determine whether the recent increases were due to changes in maternal characteristics or obstetric practice.
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MATERIALS AND METHODS |
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Because our objective was restricted to examining the determinants of primary cesarean delivery, we excluded all women with a previous cesarean delivery. Deliveries constituted the unit of observation, and women with twin (or higher-order) pregnancies were only counted once (the method of delivery of the second twin was used in the analysis). All trends over time were estimated by year and also by period (1988–1991, 1992–1994, 1995–1997, and 1998–2000). The magnitude of the change in primary cesarean delivery rates over time was estimated by use of relative risks, relative risk reductions, and 95% confidence intervals (CIs).
To understand the contribution of changes in maternal characteristics and obstetric practice factors to increases in primary cesarean delivery rates, we first carried out individual-level regression analyses with logistic regression. We categorized potential determinants of primary cesarean delivery into several groups: maternal characteristics, maternal conditions or diseases, factors related to obstetric practice, and fetal or infant characteristics. We reasoned that changes in maternal characteristics (eg, increases in older maternal age) and maternal conditions or diseases (eg, multifetal pregnancy) could lead to changes in obstetric practice (eg, increases in induction rates). We therefore used sequential models to identify the effect of each factor and each group of factors on the relationship between year or period and primary cesarean delivery.
The maternal characteristics considered to be potential determinants of cesarean delivery included maternal age (less than 20 years, 20–29, 30–34, 35–39, and 40 or more years), parity (0, 1, 2, 3–4, and 5 or more), prepregnancy weight (less than 55 kg, 55–59, 60–69, and 70 or more kg), weight gain during pregnancy (less than 5 kg, 5–9, 10–14, 15–19, and 20 or more kg), and maternal smoking during pregnancy (nonsmoker, 1–9 cigarettes per day, and 10 or more cigarettes per day). We also adjusted for maternal conditions or diseases, such as hypertensive disorders in pregnancy (including severe pregnancy-induced hypertension; hemolysis, elevated liver enzymes, low platelets syndrome; chronic hypertensive disease; and eclampsia). Other maternal conditions or diseases adjusted for included prepregnancy or gestational diabetes (yes or no), multiple pregnancy (yes or no), previous fetal death (yes or no), and previous neonatal death (yes or no). Preterm (less than 37 weeks) induction, term (37–40 weeks) induction, postterm (41 or more weeks) induction, epidural anesthesia, and delivery by an obstetrician (an obstetrician is the attending physician for all high-risk and some low-risk deliveries in Nova Scotia) were the obstetric practice factors identified as potential determinants of primary cesarean delivery. Finally, we also adjusted for fetal or infant characteristics: gestational age, small for gestational age,13 and birth weight. Time trends were estimated both for individual year and period through separate logistic regression models.
In adjusting for changes in obstetric practice, we were unable to control for potential declines in midpelvic forceps delivery and forceps delivery after rotation of the fetal head. This was because such delivery and primary cesarean delivery are mutually exclusive and cannot be modeled in individual-level logistic regression analysis (ie, with midpelvic forceps or rotational deliveries represented as potential determinants of primary cesarean delivery). We therefore carried out an ecologic Poisson regression analysis14 of primary cesarean delivery rates in the 18 counties of Nova Scotia. The dependent variable in this Poisson model was the number of primary cesarean deliveries. The data were stratified by year (13 levels) and county (18 levels), which resulted in 234 strata. The other independent variables considered for inclusion in the model were based on the determinants considered for the logistic model.
The odds ratios obtained from the logistic models were converted to relative risks15 to facilitate interpretation (because outcomes rates were typically 10% or more). Numeric concerns with model fitting (collinearity) were considered present if unexpectedly large standard errors were encountered.16 All statistical tests were two-sided.
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RESULTS |
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). Primary cesarean delivery rates increased significantly from 13.1% between 1988–1991, to 13.6% between 1992–1994, 14.2% between 1995–1997, and 15.8% between 1998–2000. The pattern of the increase was approximately similar in the four geographic regions of Nova Scotia.
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, Figure 1). Obstetric practice also changed, with large increases in labor induction rates and use of epidural anesthesia (Figure 1). The frequency of all midpelvic forceps deliveries decreased from 7.5% between 1988–1991 to 2.0% between 1998–2000 (midpelvic forceps deliveries decreased from 3.3% to 1.4%, respectively whereas low-midpelvic forceps deliveries decreased from 4.2% to 0.6%, respectively) and deliveries after rotation of the fetal head (forceps or manual) declined from 4.1% between 1988–1991 to 3.7% between 1998–2000. There was no change in the rate of primary cesarean delivery after attempted forceps delivery (approximately 0.6% of deliveries). The frequency of low birth weight (less than 2500 g) births decreased from 5.5% to 5.0%, whereas birth weights 4000–4499 g increased from 12.4% to 13.5%, and birth weights 4500 or more g increased from 2.7% to 2.9% from 1988–1991 to 1998–2000.
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). There was an 8%, 25%, 17%, and 7% increase in primary cesarean deliveries between 1988–1991 and 1998–2000 among births less than 2500 g, 2500–3999 g, 4000–4499 g, and 4500 or more g, respectively. Nonsignificant declines were observed in primary cesarean delivery rates among teenage mothers and after preterm labor induction (Table 3). Primary cesarean delivery rates declined significantly among those receiving epidural anesthesia (P < .001) but increased among those not receiving epidural anesthesia (P <.001).
Controlling for maternal age in the individual-level analyses explained a small proportion of the 21% period increase in primary cesarean delivery rates (Table 4). Simultaneous adjustment for maternal age and parity explained a substantial proportion of the increase in primary cesarean delivery rates between 1988–1991 and 1998–2000 (relative risk 1998–2000 versus 1988–1991 = 1.13, 95% CI 1.09, 1.18; Figure 2). The observed period increase in primary cesarean delivery rates was entirely explained by adjustment for changes in maternal age, parity, prepregnancy weight, and weight gain in pregnancy (relative risk 1998–2000 versus 1988–1991 = 1.02, 95% CI 0.97,1.06; Table 3 and Figure 2); additional adjustment for maternal smoking did not alter this (Table 4).
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Table 5 presents the results of the final regression model and shows the relationship between each maternal characteristic, maternal condition or disease, and obstetric practice factor and primary cesarean delivery. Modification of the effect of epidural anesthesia on primary cesarean delivery (over calendar period) was confirmed in the final logistic model (P for interaction term < .001). Additional adjustment for fetal or infant characteristics did not change the period effects materially.
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Ecologic Poisson regression modeling also showed that the temporal increases in primary cesarean delivery rates could be entirely explained by changes in maternal characteristics, namely, age, parity, and weight gain during pregnancy (relative risk for 2000 versus 1988 = 1.07, 95% CI 0.94, 1.21). Compared with the model that only included maternal characteristics, the model that included maternal characteristics, maternal conditions or diseases, and obstetric practice factors showed a larger temporal reduction in primary cesarean delivery rates (relative risk for 2000 versus 1988 = 0.96, 95% CI 0.82, 1.11). An absolute 1% decrease in midpelvic forceps delivery rates (either midpelvic or low-midpelvic) was associated with a relative 2% increase (95% CI 1%, 3%, P = .001) in primary cesarean delivery.
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DISCUSSION |
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).
It is not surprising that changes in maternal characteristics explained the temporal increase in primary cesarean delivery rates. Older maternal age,17–19 reduced parity,4,20 high prepregnancy weight,4,20–24 and increased weight gain in pregnancy21 are documented risk factors for cesarean delivery. Interpreting the role of changes in obstetric practice presents a challenge, however, because these may have occurred as a consequence of changes in maternal characteristics or maternal conditions or diseases. Older maternal age, for instance, is associated with higher rates of hypertension, diabetes mellitus, preeclampsia, and other antenatal complications and higher rates of labor induction.17–19 Our study showed a modest association between epidural anesthesia and primary cesarean delivery (adjusted relative risk 1.16, 95% CI 1.12, 1.20; Table 5). This association is generally consistent with the results of randomized trials, which show that epidural anesthesia is consistent with no increase, or at most a small increase in cesarean delivery.25 The temporal reduction in primary cesarean delivery rates among those receiving epidural anesthesia in our study probably occurred because such analgesia was more often available to the higher-risk women delivering in tertiary or large hospitals in the early years of our study.
Changes in obstetric practice have also occurred because of concerns related to fetal safety and maternal morbidity. In 1994, the American College of Obstetricians and Gynecologists recommended that midpelvic forceps deliveries be performed only in rare emergencies and with simultaneous preparation for cesarean delivery.26 This change in obstetric practice was based on studies related to long-term infant morbidity.27 Concerns related to perineal damage from vaginal delivery, including subsequent stress incontinence and anal sphincter damage28,29 have probably also contributed to increases in primary cesarean delivery. Finally, increases in primary cesarean delivery for breech presentation occurred between 1988–1991 and 1992–1994 and again between 1995–1997 and 1998–2000 (Table 2), much before compelling evidence in favor of cesarean delivery became available.30
Our findings are probably generalizable (to a greater or lesser extent) to other regions of Canada, as well as to other industrialized countries in which primary cesarean delivery rates have increased. Live births to older women have increased in most industrialized countries1,2,4; 14.3% of live births in Canada in 19971 and 13.5% of live births in the United States in 20002 were to women 35 or more years old (compared with 12.3% in Nova Scotia in 2000). Temporal increases in prepregnancy weight and weight gain in pregnancy in the United States also seem to be similar to the changes described in our study. The proportion of women with a weight of 200 or more lb (approximately 91 kg) at the first prenatal visit in Jefferson County, Alabama, increased from 7.3% in 1980 to 24.4% in 2000,22 whereas 18.7% of women who delivered in the United States in 1999 gained 41 or more lb (approximately 19 kg) during pregnancy.2 Increases in labor induction rates in Canada1 and the United States2,31 have also been similar to those observed in Nova Scotia. Some differences in primary cesarean delivery rates should be noted, however. Primary cesarean delivery rates declined in Nova Scotia in the early 1990s before rising from 1994 onward (Figure 2). Elsewhere in Canada and in the United States, however, the decline in primary cesarean delivery rates continued into the mid-1990s, and increases in primary cesarean delivery rates did not begin until 1996 and 1997, respectively.
Our study did not attempt to determine the "ideal" cesarean delivery rate. Primary cesarean delivery rates in Nova Scotia currently exceed the World Health Organization’s prescriptive rate of 10–15% for overall cesarean delivery.5 In this context, it may be instructive to recall the surge in cesarean delivery rates that occurred in the late 1960s and 1970s. It has been suggested that increases in cesarean deliveries and the advent of neonatal intensive care were causally responsible for the dramatic declines in perinatal mortality during this period.32 On the other hand, "abusive indications for elective cesarean delivery"33 can contribute to both maternal and infant morbidity.
The limitations of our study include those that are typical of large databases. Although the Nova Scotia Atlee Perinatal Database has a system for ensuring data integrity (including monitoring and abstraction studies), and no significant coding practice changes occurred during the study period, some transcription and other minor errors are inevitable. The loss of subjects from the logistic regression model because of missing values is a potential concern, although supplementary analyses suggest that the extent of any potential bias is likely to be small. Finally, the conversion of odds ratios obtained from logistic regression models to relative risks involves some approximations, which we believe are justified by gains in interpretability.
In summary, the substantial recent increases observed in primary cesarean delivery can be explained by concurrent changes in maternal age, parity, prepregnancy weight, and weight gain during pregnancy. Obstetric practice, which has changed owing to changes in maternal characteristics and concerns related to fetal and maternal safety, has also contributed to increases in primary cesarean delivery. Calls for reducing primary cesarean delivery rates and especially target-driven restrictions on primary cesarean delivery should be tempered by an understanding of temporal changes in maternal characteristics and the rationale behind changes in obstetric practice.
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Footnotes |
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The authors thank the Reproductive Care Program of Nova Scotia for providing access to data.
doi:10.1016/S0029-7844(03)00620-3
Received February 4, 2003. Received in revised form April 9, 2003. Accepted May 2, 2003.
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REFERENCES |
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