Cesarean delivery has increased in the United States over the past decade with current rates approaching one in three deliveries occurring by cesarean delivery.1 This increase has resulted in focused discussions surrounding current indications for cesarean delivery in hopes of curtailing this trend. Examples of these include the study of vaginal trial of labor after cesarean delivery, elective inductions of labor before 39 weeks of gestation, reclassification and management recommendations for intrapartum electronic fetal monitoring, and revisiting the efficacy of external cephalic version near term for fetuses in breech presentation.2 – 10 Despite recent data, a clear rationale for the dramatic increase in cesarean delivery has not been identified, and whether this rise has resulted in improvements in maternal or neonatal outcomes remains unclear. In a study that aimed to identify potential reasons for the recent increase in primary cesarean delivery rates, 16% of the rise was attributable to multiple gestation.11 Because cesarean delivery can increase both short-term and long-term maternal morbidity, justifications for its routine use such as potential neonatal benefit are warranted.12
Recently, the optimal mode of delivery has been a point of debate for twin gestations, particularly in regard to its effect on neonatal outcomes.13 – 20 Although most health care providers feel comfortable delivering vertex–vertex twins vaginally, fewer are comfortable with breech delivery of the second twin in cases of vertex–nonvertex twins despite data suggesting similar neonatal outcomes in experienced hands.14,21 Epidemiologic studies have shown a potential benefit of cesarean delivery when the second twin is nonvertex, and it has been suggested that this situation may be amenable to counseling patients of the risk–benefit in a similar fashion as in singleton breech presentation or a trial of vaginal delivery after cesarean delivery.22 – 24 Ultimately, there is not clear evidence base for routine cesarean delivery for twin gestation.25
A previous report of U.S. twin births demonstrated a relatively high cesarean delivery rate for twin gestation, which rose modestly from 50% to 56% from 1995 to 1998.26 On the other hand, a decrease in cesarean delivery for twins was seen in France from 50% in 2000–2001 to 36% in 2006.27 The recent secular trend in cesarean delivery rates for twin gestations in the United States has not been characterized. The objective of our study was to estimate trends in cesarean delivery rates for twin births in the United States over the past two decades and to estimate risk factors associated with cesarean delivery in twins during this time period.
MATERIALS AND METHODS
A cross-sectional population study using the U.S. birth cohort data sets from 1995 to 2008 as collected by the National Center for Health Statistics from all 50 states and the District of Columbia was performed.
The publicly available U.S. birth certificate records during this study period note whether the gestation was singleton, twin, or higher order but do not allow for matching of siblings. Therefore, we considered each neonate as a single unit of analysis. Because twins are not matched, the order of presentation and the presentation of the corresponding twin is unavailable. The U.S. birth certificate is only recorded for live births, and therefore, twin gestations that ended with one twin stillborn would only have one twin reported in birth certificate data.
The primary outcome was mode of delivery, which was characterized as either vaginal or cesarean birth. The main predictor variable of interest was the year of birth. We also considered fetal presentation (defined as “vertex” or “breech” by the certificate data), fetal distress, cephalopelvic disproportion, preterm birth (less than 37 weeks of gestation), small for gestational age (defined as less than the 10th percentile according to published norms),28 prior cesarean delivery, induction of labor, and maternal factors including hypertensive disease and diabetes as variables. For some states in earlier years of data collection, “breech” also was categorized as “breech or malpresentation.”
Cesarean delivery rates were calculated for all twin births, vertex and breech presentation by year of delivery, and by maternal and neonatal characteristics noted previously. Univariable analysis using the chi-square test and multivariable analysis using logistic regression were used to estimate independent risk factors for cesarean delivery. For each medical risk factor, we tested whether there was a significant change across the study period by including year as a linear variable in logistic regression in an analysis stratified by that risk factor. To test the difference in the trend between those that did or did not have a medical risk factor, the logistic model included the following predictor variables: the medical risk factor, year, and the interaction between the risk factor and year. Statistical significance was determined by P<.05. For 2007 and 2008, the variables noting fetal distress or cephalopelvic disproportion were no longer recorded on the standard birth certificate, and therefore, those 2 years were not included in the multivariable logistic regression analyses.
Risk factors were included in a multivariable logistic regression model with mode of delivery as an outcome, and odds ratios with 95% confidence intervals were estimated. This was completed for years 1995–2006 as a whole with year as a linear variable and then separately for each of 1995 and 2006. We converted odds to relative risk or risk ratio using the method outlined by Zhang.29 Because we could not directly adjust for the statistical effects of clustering within twin pairs, we performed a sensitivity analysis by inflating the standard errors in our analyses upward by a factor of 1.41, a maximally conservative adjustment that assumes nearly perfect levels of intratwin correlation.
This study was considered exempt from review by the University of California, San Francisco Committee on Human Research. All statistical analyses were completed using SAS 9.2.
There were 1,702,365 twins born in the United States from 1995 to 2008 representing 3.0% of all live births. In this cohort, the proportion of records with unknown delivery mode was 9,034 (0.53%). Maternal demographic characteristics with group-specific cesarean delivery rates for twin births are shown in Table 1. The proportion of twins from multiple gestation pregnancies increased from 2.5% of all births in 1995 to a plateau of 3.4% from 2007 to 2008 (Table 2). Cesarean delivery rates were stable at 53.4%–53.9% from 1995 to 1997 and then increased steadily up to 75.0% in 2008 (Table 2; Fig. 1). Over the 14-year period, cesarean delivery for twins in the breech presentation increased from 81.5% to 92.1% and for those in the vertex presentation from 45.1% to 68.2% (Fig. 1).
Cesarean delivery rates increased 38.4% for twins born at term and increased 39.5% for twins born preterm (P<.001 for both trends and for difference between groups). The largest relative increases in cesarean delivery occurred for twins with one or more of the following factors: no previous cesarean delivery, vertex presentation, without fetal distress or cephalopelvic disproportion, and nondiabetic mother (Table 3). In sensitivity analysis in which the maximum potential effect of clustering at the twin pair level was accounted for, all results remained statistically significant except for the yearly trend for neonates with cephalopelvic disproportion.
After risk adjustment, we found that there was an average 5% increase in cesarean delivery rate during each year of the study period (risk ratio 1.05, 95% confidence interval 1.04–1.05). In comparing the predictors for mode of delivery in twin gestations over the time interval, risk factors such as hypertension, diabetes, cephalopelvic disproportion, fetal distress, and previous cesarean delivery continued to increase the risk for cesarean delivery, but the magnitude of their overall effect decreased over time (Table 4). On the other hand when analyzing trends in the lowest risk group, those lacking any of the risk factors identified previously, the cesarean delivery rate rose from 33.6% in 1995 to 56.6% in 2006.
The cesarean delivery rate for twin gestations in the United States had increased modestly in the years just before the current study period, from 50% to 53% over the years 1989–1994.26 We found a substantially more dramatic and steady rise in cesarean delivery rates from 1995 to 2008. Cesarean delivery rates for twins in breech presentation were already above 80% at the beginning of the study period and rose to greater than 90%. The relative increase in cesarean delivery for twins in vertex presentation was dramatically higher, increasing more than 50% from 45.1% to 68.2%.
We were unable to explain this increase in cesarean births for twins on the basis of higher rates of obstetric and medical complications necessitating operative delivery during the period analyzed. We found that the highest relative increases in cesarean delivery rates occurred in what would usually be considered lower risk conditions such as no fetal distress and no cephalopelvic disproportion (Table 3). After risk adjustment for factors such as hypertension, diabetes, and prematurity, there was a 5% increase in cesarean delivery each year.
When considering singletons in breech presentation, studies have tended to show a potential benefit for cesarean delivery, yet the American College of Obstetricians and Gynecologists allows for experienced practitioners to consider vaginal breech delivery in certain circumstances, while recognizing that cesarean delivery may be preferred in general.24 It is possible that some of the increase in twin cesarean delivery may be attributable to what may be considered appropriate use of cesarean delivery for breech presentation of either the presenting or second twin.15,22,23 We were not able to account for order of presentation in our study and therefore could not refine our analysis to answer this specific question. Because the proportion of twins in breech presentation remained relatively low, ranging from 23% to 28%, we suspect that the cesarean delivery rate for twins in any combination of presentations, including vertex–vertex presentation, increased significantly during the study period. A study of U.S. twin births that occurred at 34 weeks of gestation or more found that, when the first twin presented in the vertex position, 87% of second twins were also vertex, so that the vertex–vertex combination comprised the majority of presentations.21
Although birth certificate data do not characterize the order of presentation in twin gestations, we can estimate the approximate contribution of first twin breech presentation to what may be considered appropriate cesarean delivery of the second twin in vertex presentation. Knowing that approximately 75% of twins in the data set are vertex, if we estimate that 20% of twin pregnancies have first twin breech,16,30 we would expect that less than 10% of the vertex twins in this analysis would have been delivered by cesarean as a result of the first twin being breech. We found that by 2008, 68.2% of vertex twins were delivered by cesarean. If we presumed that 10% of these were second twins after a first twin in breech position and that 100% of these were delivered by cesarean, the cesarean delivery rate for vertex–vertex would still be 64.7%.
In prior studies of singletons and twins, induction of labor has been variably found to have positive, inverse, and no association with cesarean delivery, depending on the circumstances of induction and gestational age.31 – 34 We found that induction of twin gestation pregnancies increased from 11.3% in 1995 to 13.8% for 1998–1999 and then steadily decreased to 9.7% to 9.9% in 2007 and 2008. The recent decline in inductions might indicate that some obstetricians were more inclined to proceed straight to cesarean delivery as opposed to a trial of labor in some circumstances. Although induction of labor was associated with significantly lower rates of cesarean than noninduced deliveries, there was still an increase in the rate of cesarean delivery for induced deliveries from 26.3% to 32.5% during the study period.
We considered that the increasing trend in cesarean delivery overall could have contributed to the trend for twins, because women who had a previous cesarean delivery are more likely to undergo cesarean delivery for subsequent pregnancies. Indeed, previous cesarean delivery was one of the strongest risk factors associated with cesarean delivery in this cohort of twin deliveries. However, this could only have been a partial contributor, because the large majority of deliveries occurred in women without a previous cesarean delivery throughout the study period. Neonates born to women with a prior cesarean delivery rose from 12.4% in 1995 to 14.9% in 2008.
Maternal morbidity associated with cesarean delivery, including peripartum infection and bleeding, is largely influenced by unplanned cesarean deliveries occurring during a trial of labor. It has been suggested that elective cesarean delivery for twins at 37 weeks could potentially reduce maternal morbidity by decreasing the need for unplanned cesarean deliveries.20 Although overall planned cesarean deliveries have fewer adverse maternal effects than unplanned cesarean deliveries, planned cesarean deliveries still confer longer hospital stays, higher bleeding, and infection rates when compared with planned vaginal deliveries.35,36 Maternal morbidity data for planned cesarean delivery compared with planned vaginal delivery in multiple gestations remains limited. In a French study, women undergoing vaginal delivery were more likely to have postpartum hemorrhage than planned cesarean deliveries, but composite morbidity was not different between groups.16 Given their overall low frequency, larger studies are necessary to adequately study adverse maternal outcomes among women with multiple gestations. Regardless, cesarean delivery does increase abnormal placentation, including placenta previa and accreta, with future pregnancies.35
A limitation of our analysis was an inability to characterize cases in which the first twin delivered vaginally and the second by cesarean. Previous study of U.S. birth certificates has shown that this may occur up to 4%–6% of the time.19,21 Because those studies were performed on births during 1995–1997, we would presume that this occurrence would have decreased in frequency over time with the increase in cesarean delivery. Furthermore, for those cases in which there was an initial vaginal delivery followed by cesarean delivery, our study would have counted this as one vaginal and one cesarean birth. For an analysis at the maternal level, this would count as one cesarean delivery, and therefore it is possible that our study would be an underestimate of cesarean delivery rates. We also did not have data on births in which there may have been stillbirth of a cotwin. It is unclear how incorporation of these data would have affected the results of this study. A further limitation of this analysis is that some conditions that were considered low-risk may have had other risk factors that were not listed or considered in our analysis.
The strength of our study is that this is not a sampling, but represents the entire U.S. population over a 14-year period. Although twins are a relatively small proportion of all births, the number of twin gestations increased 36% during the study period. The number of twin births in 2008 was 145,175, and 75% of those twins being delivered by cesarean would translate to more than 50,000 women having undergone cesarean delivery in that year.
As the cesarean delivery rate for twin gestation continues to increase, it is possible that the skills required for vaginal delivery of multiples may be lost by clinicians. Because it appears that cesarean delivery has now become the norm, an increasing number of clinicians may opt to avoid vaginal delivery for a variety of reasons, including medicolegal as well as comfort level. Training and practice for vaginal delivery of singleton breech neonates can occur with vaginal delivery of the second twin in breech position. Now this opportunity has become very rare, because less than 10% of breech twins are delivered vaginally.
There is a growing interest in stemming the rise in cesarean delivery rates for all pregnancies.37,38 In that context, we note that cesarean delivery for most twin gestations, especially those in vertex–vertex presentation, has no proven clinical benefit for either mother or child. The dramatic rise in cesarean delivery rates for twins undoubtedly has adverse implications for maternal morbidity and health care costs. Curtailing elective cesarean deliveries in this cohort may prove to be beneficial for both the individual woman and society at large.
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