The Cesarean delivery rate for twin gestations in the United States had increased modestly in the years just prior to the current study period, from 50% to 53% over the years 1989 to 1994.(26
) We found a substantially more dramatic and steady rise in cesarean delivery rates from 1995 to 2008. Cesarean 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 rates occurred in what would usually be considered lower risk conditions, such as no fetal distress and no cephalopelvic disproportion (). 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
) 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. As the proportion of twins in breech presentation remained relatively low, ranging from 23% to 28%, we suspect that the cesarean 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 were 34 weeks gestation and greater found that when the first twin presented in 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 does 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 dataset are vertex, if we estimate that 20% of twin pregnancies have first twin breech,(16
) we would expect that < 10% of the vertex twins in this analysis would have been delivered by cesarean due to 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 following a first twin in breech position and that 100% of these were delivered by cesarean, the cesarean 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
) We found that induction of twin gestation pregnancies increased from 11.3% in 1995 to 13.8 % for 1998–1999, 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 non-induced deliveries, there was still an increase in the rate of cesarean 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, as women who had a previous cesarean delivery are more likely to undergo cesarean delivery for subsequent pregnancies. Indeed, previous cesarean 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, as the large majority of deliveries occurred in mothers without a previous cesarean throughout the study period. Neonates born to mothers with a prior cesarean 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 cesareans 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 cesareans.(20
) Though overall planned cesarean deliveries have fewer adverse maternal effects than unplanned cesareans, planned cesareans still confer longer hospital stays, higher bleeding, and infection rates when compared with planned vaginal deliveries.(35
) Maternal morbidity data for planned cesarean delivery versus 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 in order 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
) As those studies were performed on births during 1995 to 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, 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 rates. We also did not have data on births in which there may have been stillbirth of a co-twin. It is unclear how incorporation of this data would have impacted the results of this study. A further limitation of this analysis is that some conditions which were considered as “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 United States 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 mothers having undergone cesarean delivery in that year.
As the cesarean rate for twin gestation continues to increase, it is possible that the skills required for vaginal delivery of multiples may be lost by clinicians. As 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 medico-legal, as well as comfort level. Training and practice for vaginal delivery of singleton breech infants can occur with vaginal delivery of the second twin in breech position. Now this opportunity has become very rare, as 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
) 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 cesareans in this cohort may prove to be beneficial for both the individual mother and society at large.