Objective
The effect of a cesarean delivery in different stages of labor on spontaneous preterm birth (sPTB) in a subsequent pregnancy has not been studied extensively. The objective of the study was to evaluate the risk of subsequent sPTB after a first-stage or second-stage cesarean delivery compared with a vaginal delivery.
Study Design
This was a planned secondary analysis of a large retrospective cohort study of women with 2 consecutive deliveries from 2005-2010. Women with a previous sPTB were excluded. First-stage (<10 cm) and second-stage (≥10 cm) cesarean deliveries were compared with vaginal deliveries. Data were obtained through chart abstraction. The primary outcome was sPTB (<37 wk) in a subsequent pregnancy. Categoric variables were compared with the use of χ 2 analyses, and logistic regression was used to calculate odds and control for confounders.
Results
Eight hundred eighty-seven women were included (721 vaginal deliveries; 129 first-stage and 37 second-stage cesarean deliveries). The sPTB rate varied between groups (7.8%, 2.3%, and 13.5%, respectively; P = .03). When compared with women with a vaginal delivery, women with a first-stage cesarean delivery had a decreased risk of sPTB, which remained after adjustment for confounders (adjusted odds ratio, 0.30; 95% confidence interval, 0.09–0.99; P = .049). There was a nonsignificant increase in odds of sPTB after a second-stage cesarean delivery compared with a vaginal delivery (adjusted odds ratio, 2.4; 95% confidence interval, 0.77–7.43; P = .13). Women with a second-stage cesarean delivery had a 6-fold higher odds of sPTB compared with women with a first-stage cesarean delivery, which remained after adjustment for confounders (adjusted odds ratio, 5.8; 95% confidence interval, 1.08–30.8; P = .04).
Conclusion
Women with a full-term second-stage cesarean delivery have a significantly higher than expected rate of subsequent sPTB (13.5%) compared with both the overall national sPTB rate (7-8%) and to a first-stage cesarean delivery (2.3%). As the cesarean delivery rate continues to rise, this potential impact on pregnancy outcomes cannot be ignored.
Although the rate of preterm birth (PTB) has decreased since 2006, the decline has been marginal. PTB remains a large public health concern and a contributor to neonatal morbidity. The overall PTB rate in the United States is currently 11.7%, with a spontaneous PTB (sPTB) rate of 7-8%. There are many known risks for PTB ; however, most women do not have an identified risk factor at first evaluation.
One possible risk factor that has not been studied extensively is the effect of a previous cesarean delivery in the second stage of labor. The cervical trauma from a prolonged second stage of labor and injury of the cervix during a cesarean delivery are events that could alter the cervical integrity and place women at risk for PTB in a subsequent pregnancy. For example, unintentionally incising the cervix during uterine incision or cervical extensions and lacerations during delivery may disrupt cervical integrity and compromise the cervical function for future pregnancies. This theory has been suggested by expert opinion and described in case reports ; however, the effect of a second-stage cesarean delivery on PTB has not been evaluated scientifically.
Therefore, the objective of the study was to evaluate the risk of subsequent sPTB after a cesarean delivery in the first and second stage of labor compared with women who had a vaginal delivery. Our hypothesis was that women with a cesarean delivery in the second stage of labor are at an increased risk of sPTB in a subsequent pregnancy, compared with women who had a vaginal delivery and compared with women who had a cesarean delivery in the first stage of labor.
Materials and Methods
This study was a planned secondary analysis of a large retrospective cohort study that was designed to evaluate the impact of induction of labor on sPTB in a subsequent pregnancy. The cohort included women with 2 consecutive deliveries at the Hospital of the University of Pennsylvania during the years 2005-2010. The current study uses this entire cohort to evaluate the effect that a cesarean delivery in one pregnancy has on the risk of sPTB in a subsequent pregnancy. Approval from the institutional review board was obtained before the study.
Patient selection
Using our hospital-based electronic database, we were able to obtain a list of patients who had >1 delivery from 2005-2010. The first pregnancy during this time period was considered their ‘index’ pregnancy. The index pregnancy may or may not equate to the patient’s first pregnancy because multiparous women were included among index pregnancies. The second pregnancy during this time period was then considered a subsequent pregnancy. Patients were included in the study only once.
As in the parent study, this study included women whose index pregnancy was at term (≥37 weeks of gestation) and who had a consecutive subsequent delivery at ≥16 weeks of gestation at our institution. Women who were preterm in their index pregnancy or those with a PTB history at the time of their index pregnancy were excluded, given their high a priori risk of a subsequent PTB. Women who underwent a trial of labor after cesarean delivery were included; however, women with a planned or scheduled cesarean delivery were excluded.
Exposure definition
The exposed group included women who had had a cesarean delivery in their index pregnancy; this group was subdivided into women who had had a cesarean delivery in the first stage of labor (defined as a cesarean delivery at <10 cm dilation) and women who had a cesarean delivery in the second stage of labor (defined as a cesarean delivery at 10 cm dilation) in the index pregnancy. The unexposed group included women who had had a vaginal delivery in the index pregnancy.
Outcomes definition
The primary outcome was sPTB in a subsequent pregnancy that was defined as spontaneous labor and delivery or preterm premature rupture of membranes at <37 weeks of gestation. The secondary outcomes were sPTB at <34 weeks of gestation in a subsequent pregnancy and overall PTB, which included both medically indicated PTB and sPTB at <37 weeks of gestation.
Estimated gestational age at the time of delivery was based on standard obstetric dating, as described in the primary article. For both the index pregnancy and subsequent pregnancy, if a patient had unknown dating (n = 12; 1.4%), an ultrasound scan was obtained before delivery to confirm whether it was preterm or term. In those patients for whom an ultrasound scan was unable to be performed before delivery (n = 4), a birthweight of >3000 g confirmed a term gestation. Gestational age then was confirmed subsequently by a pediatric examination in all patients with unknown dating.
Data collection
Data collection was through chart abstraction from the maternal and neonatal electronic medical records. Variables that were collected included maternal demographics and a full obstetric, gynecologic, medical, and social history. Labor and delivery information that was obtained included the lengths of the latent phase, active phase, and the second stage of labor. Delivery information that was abstracted included mode of delivery and neonatal information, for both the index and subsequent pregnancies.
Data analysis
Our analysis occurred in 2 stages. The first part of the analysis compared demographic data among all groups. Tests for skewness/kurtosis were performed to examine normality of continuous variables. Kruskal-Wallis tests were used to compare data that did not meet the assumptions of normality. Analysis of variance was used to compare normally distributed data. Pearson χ 2 and Fisher exact tests were used to compare categoric variables, as appropriate.
The second part of the analysis focused on our objective, which was to evaluate the risk of subsequent sPTB after a cesarean delivery in the first and second stages of labor compared with women who had had a vaginal delivery. We used bivariate comparisons to assess for potential confounders or risk factors for the outcome of sPTB. Risk factors that were associated with the dependent variable with a significance level of < .2 were considered candidate risk factors and/or potential confounders for multivariable modeling that included maternal age, body mass index, race, chronic hypertension, parity, previous cesarean delivery, interpregnancy interval, and induction of labor in the index pregnancy. We then created our multivariable model and used a backwards stepwise elimination strategy to obtain a parsimonious model. Any risk factor that impacted the effect size between exposure status and outcome by at least 15% was retained in the model as a confounder. The confounders that were included in all of the final models were race, chronic hypertension, and induction of labor in the index pregnancy.
Data analysis was performed with STATA software (version 12.0 for Windows; STATA Corporation, College Station, TX). Statistical significance was set at a probability value of < .05. The parent study had a fixed sample size of 887 women. The incidence of sPTB at <37 weeks of gestation among those who had had a vaginal delivery during their index pregnancy (unexposed group) was 7.8%. A post-hoc power analysis showed >80% power to detect an odds ratio (OR) of 3.6 for the association between second-stage cesarean delivery and sPTB.
Results
Eight hundred eighty-seven women met the inclusion criteria and were evaluated. The cesarean delivery rate for the index pregnancy was 18.8% overall. Table 1 shows the demographic information for those with a vaginal delivery, those with a cesarean delivery in the first stage of labor, and those with a cesarean delivery in the second stage of labor in the index pregnancy. Demographic information differed among the 3 groups for all characteristics, except the presence of gestational diabetes mellitus, pregnancy-related hypertension (defined as gestational hypertension or preeclampsia), no prenatal care in the subsequent pregnancy, and the interpregnancy interval.
| Variable | Delivery method | P value | ||
|---|---|---|---|---|
| Vaginal (n = 721) | First stage cesarean (n = 129) | Second stage cesarean (n = 37) | ||
| Maternal age, y a | 23 (19–29) | 22 (19–29) | 30 (24–34) | < .001 |
| Maternal age categories, % (n) | ||||
| <18 Y | 8.6 (62) | 10.0 (13) | 0 | .007 |
| 18-35 Y | 84.6 (609) | 81.5 (106) | 78.4 (29) | |
| >35 Y | 6.8 (49) | 8.5 (11) | 21.6 (8) | |
| Body mass index, kg/m 2 a | 25.8 (22.1–30.6) | 28.1 (24.2–33.2) | 24.9 (22–26.8) | .003 |
| Body mass index categories, % (n) | .049 | |||
| Underweight | 3.6 (25) | 1.6 (2) | 2.7 (1) | |
| Normal weight | 40.9 (284) | 27.3 (35) | 48.7 (18) | |
| Overweight | 28.2 (196) | 35.2 (45) | 27.0 (10) | |
| Obese | 27.3 (190) | 35.9 (46) | 21.6 (8) | |
| Gestational age at delivery, wk b | 38.8 (2.5) | 39.0 (1.6) | 38.4 (3.3) | .25 |
| Race, % (n) | ||||
| Black | 76.7 (527) | 83.7 (103) | 48.6 (17) | < .001 |
| White | 16.2 (111) | 8.9 (11) | 31.4 (11) | |
| Other | 7.1 (49) | 7.3 (9) | 20 (7) | |
| Medical comorbidities, % (n) | ||||
| Chronic hypertension | 2.4 (17) | 7.7 (10) | 5.4 (2) | .005 |
| Gestational diabetes mellitus | 1.3 (9) | 4.0 (5) | 2.7 (1) | .43 |
| Pregnancy-related hypertension | 16.5 (119) | 20.0 (26) | 27.0 (10) | .46 |
| Parity, % (n) | < .001 | |||
| Nulliparous | 59.4 (428) | 77.7 (101) | 75.7 (28) | |
| Multiparous | 40.6 (292) | 22.3 (29) | 24.3 (9) | |
| No prenatal care in subsequent pregnancy, % (n) | 6.4 (46) | 6.9 (9) | 0 | .27 |
| Previous cesarean delivery, % (n) | 1.4 (10) | 6.9 (9) | 2.7 (1) | < .001 |
| Interpregnancy interval, y a | 1.3 (0.79–2.27) | 1.4 (0.89–2.41) | 1.3 (0.72–1.86) | .09 |
| Induction of labor in index pregnancy, % (n) | 66.2 (477) | 87.6 (113) | 86.5 (32) | < .001 |
| Birthweight in index pregnancy, g b | 3261.6 ± 478 | 3270.8 ± 572 | 3548.6 ± 533 | .02 |
| Neonatal intensive care unit admission in index pregnancy, % (n) | 6.1 (44) | 4.6 (19) | 2.7 (1) | .009 |
| Length of second stage, h a | 0.72 (0.33–1.53) | NA | 3.08 (2.15–4.47) | < .001 |
a Data are given as median (interquartile range)
For our primary outcome, we evaluated the rate of sPTB in the subsequent pregnancy after a cesarean delivery at different stages of labor. The subsequent sPTB rate in the overall cohort was 7.2%. We evaluated the rate of subsequent sPTB after a cesarean delivery in the first stage of labor and the sPTB risk after a cesarean delivery in the second stage of labor, both compared with a vaginal delivery. As noted in Table 2 , the sPTB rate was significantly different across the 3 groups. When compared with women who had had a vaginal delivery, women with a cesarean delivery in the first stage of labor had a decreased risk of sPTB (OR, 0.28; 95% confidence interval [CI], 0.09–0.91; P = .03). This decreased risk remained after adjustment for confounders that included race, chronic hypertension, and induction of labor (adjusted OR [aOR], 0.30; 95% CI, 0.09–0.99; P = .049). There was a nonsignificant increase in the odds of sPTB after a cesarean delivery in the second stage of labor compared with a vaginal delivery (aOR, 2.4; 95% CI, 0.77–7.43; P = .13). There was no difference in the gestational age of sPTB among the 3 groups ( Table 2 ). We also evaluated the risk of subsequent sPTB among women with a second-stage cesarean delivery compared with women with a first-stage cesarean delivery. We found that women with a second-stage cesarean delivery had a 6.6-fold higher odds of sPTB compared with women with a first-stage cesarean delivery (OR, 6.6; 95% CI, 1.5–29.14; P = .01). This increased odds remained after adjustment for confounders (aOR, 5.8; 95% CI, 1.08–30.8; P = .04). There were no statistically significant differences among groups with respect to overall PTB rates and sPTB rate at <34 weeks of gestation ( Table 2 ).
