In this cohort, we identified 278 women with severe PPH and 572 matched controls. The mean EBL values were significantly higher among cases compared with controls (1685  mL vs 781  mL; P < .001). Among severe PPH cases, 209 (75.4%) women had at least 1500 mL EBL, and 157 (56.5%) women received RBC intraoperatively or within 48 hours after CD. Among all women who received RBC, 51 women received RBC intraoperatively and 122 women received RBC within 48 hours after CD, of whom 59 (48.4%) had an EBL < 1500 mL and did not receive intraoperative RBC. Maternal, obstetric, intrapartum, and perioperative characteristics are presented in Table 3. Because earlier clinical studies have observed an association between labor augmentation with PPH,11,23,29,30 we forced labor augmentation with oxytocin into the final multivariable model. Sixteen patients (10 cases; 6 controls) experienced primary neuraxial block failure before surgery (Supplemental Digital Content, Figure 2, http://links.lww.com/AA/B691). Severe PPH was more common among women who underwent CSE anesthesia and general anesthesia. Clinical factors independently associated with severe PPH are presented in Table 4. In the multivariable model, factors independently associated with severe PPH were general anesthesia (aOR = 5.4; 95% CI, 1.7–17.1), multiple pregnancies (aOR = 3.2; 95% CI, 1.7–6.3), predelivery Hb ≤ 9.9 g/dL (aOR = 3.0; 95% CI, 1.3–6.9), predelivery Hb = 10–10.9 g/dL (aOR = 2.6; 95% CI, 1.4–4.9 [reference group = predelivery Hb ≥ 11 g/dL]), multiple pregnancies (aOR = 1.8; 95% CI, 1.1–2.9), and government-assisted insurance (aOR = 1.6; 95% CI, 1.0–2.4). Compared with women who underwent primary CD, women with one previous CD had a reduced odds of severe PPH (aOR = 0.4; 95% CI, 0.2–0.9). Women aged ≥30 years had a reduced odds of PPH compared with women aged <30 years (aOR = 0.7; 95% CI, 0.5–1.0). The model had good calibration (Hosmer–Lemeshow test, P = .47) and modest discrimination (AUROC = 0.75; 95% CI, 0.71–0.78). After bootstrapping, the c-index was unaltered. In separate models, the interactions between magnesium with preeclampsia and gestational age at delivery were not statistically significant (data not presented).
Sensitivity and Subgroup Analyses
To determine whether point estimates for modes of anesthesia were influenced by women who experienced primary neuraxial block failure before surgery, we performed sensitivity analyses examining women in each CD cohort whose primary mode of anesthesia was successful. For each CD cohort, point estimates for mode of anesthesia did not appreciably change from those in the original multivariable models (data not presented).
Because general anesthesia may be considered more often for women with placental disorders, for the prelabor CD cohort, we performed an additional sensitivity analysis to assess point estimates for anesthetic techniques excluding women with abnormal placentation (n = 51) or placenta previa (n = 121). On the basis of chart review, we identified women with abnormal placentation based on information in the obstetrician’s operative note or from a placental pathology report (if available). Among those without these placental disorders, we observed comparable point estimates for anesthetic techniques to those in our original multivariable model (CSE: aOR = 3.87; 95% CI, 1.93–7.73; epidural: aOR = 1.7; 95% CI, 0.3–9.5; general anesthesia: aOR = 47.6; 95% CI, 7.26–311.9).
For women undergoing intrapartum CD, we compared PPH frequencies among those with versus without labor arrest before full cervical dilation. Severe PPH was more common among women with versus without labor arrest (132/203 (65%) vs 214/382 (56%), respectively; P = .04). Similarly, we compared PPH rates between those with versus without a prolonged second stage of labor; rates were similar in both groups (36/61 [59%] vs 99/151 [66%], respectively; P = .37).
In both CD cohorts, we assessed the frequency of women who experienced an EBL ≥ 1500 mL, intraoperative and postoperative RBC transfusion stratified by predelivery Hb categories (Supplemental Digital Content, Tables 2 and 4, http://links.lww.com/AA/B691). Among cases identified in each CD cohort, rates of intraoperative and postoperative transfusion were highest among women with a predelivery Hb level <10 g/dL. Finally, to determine whether predelivery Hb influences the magnitude of blood loss, we performed sensitivity analyses with the outcome based solely on the criteria of an EBL ≥ 1500 mL. On multivariable analyses, we observed that predelivery Hb category was not independently associated with an EBL ≥ 1500 mL in each CD cohort (data not presented).
Using detailed clinical data abstracted from 1669 medical records, we performed 2 case-control studies to identify risk factors for severe PPH during prelabor CD and intrapartum CD. Risk factors common to each CD cohort included general anesthesia, predelivery anemia, and multiple pregnancies. Other risk factors identified in our analyses were also well recognized: placenta previa (prelabor CD) and chorioamnionitis (intrapartum CD). We identified several risk factors that were less established, such as CSE anesthesia, nontransverse uterine incision, earlier D&C, and Hispanic race (prelabor CD); and gestational diabetes (GDM), young maternal age, government-assisted insurance, and primary CD (intrapartum CD). Future studies are needed to determine whether confounding by indication explains the associations between CSE and general anesthesia with severe PPH.
Although a number of studies have sought to identify risk factors for PPH during CD,15,16,21,31 the majority have not accounted for key differences in the maternal characteristics and delivery indications between prelabor and intrapartum CDs.32 Only 2 studies examined risk factors across distinct CD populations; elective/nonelective CD16 and planned/emergency CD.15 Consistent with our findings, these studies identified placenta previa and general anesthesia as key risk factors across all CD cohorts. However, neither study stratified cohorts according to the presence versus the absence of labor or induction before CD.
We believe that our approach to construct separate logistic models according to CD subtype is clinically justified for several reasons. It is not clinically intuitive to consider labor or other intrapartum characteristics as potential predictors in a single multivariable model for severe PPH during prelabor CD. Similarly, women with placenta previa are unlikely to be considered for induction or trial of labor; therefore, it would be intuitive to consider placenta previa as a candidate variable only for women who undergo prelabor CD. However, from an epidemiological standpoint, we acknowledge that future cohort studies using nuanced clinical data are needed to investigate whether interactions truly exist between relevant predictors and the presence versus the absence of labor or induction before CD. Large study populations would also be needed to construct adequately sized derivation and validation study cohorts.
In both CD cohorts, general anesthesia had the highest adjusted odds of severe PPH compared with spinal anesthesia. Because some anesthesiologists may prefer to use general anesthesia for patients with placenta previa or abnormal placentation,33 we excluded these conditions in our sensitivity analyses for the prelabor cohort and observed that the association between general anesthesia and severe PPH persisted, albeit with wider confidence intervals. This association has been reported in other observational and randomized studies, with varying degrees of magnitude.15,16,21,31,34–36 General anesthesia may itself directly influence the likelihood of severe blood loss. Volatile agents have a concentration-dependent inhibitory effect in vitro on uterine smooth muscle contractility,37,38 thereby increasing the likelihood of uterine atony. Volatile agents (halothane, sevoflurane) and induction agents (propofol) can also inhibit platelet function in a dose-dependent manner.39 However, we cannot exclude the possibility of confounding by indication because patients with anticipated risk factors for PPH may be more likely to receive general anesthesia. For example, in the prelabor CD cohort, general anesthesia was used for 21% of women with placenta previa, 46% of women with abnormal placentation, and 22% of women with antenatal vaginal bleeding (Supplemental Digital Content, Table 1, http://links.lww.com/AA/B691). In the prelabor CD cohort, women who underwent CSE anesthesia had a 3-fold increased odds of severe PPH compared with those receiving a single-shot spinal. The reason for this finding is unclear. Because equivalent doses of intrathecal local anesthetic drugs are commonly used for spinal and CSE anesthesia, it is doubtful that a drug effect explains this finding. Therefore, confounding by indication is also likely, because patients with anticipated risk factors for PPH may require longer durations of surgery and receive a CSE technique. To assess the potential for bias from confounding by indication for CSE and general anesthesia, context-specific studies are needed to examine the relations between these modes of anesthesia with severe PPH.
Predelivery anemia was associated with our primary outcome, which included a composite of major blood loss or postpartum transfusion; this association was not confirmed in our sensitivity analysis that focused on major blood loss only. These findings are consistent with those by Butwick et al40 who reported that predelivery anemia is an independent risk factor for severe postpartum anemia after CD, classified by a postpartum Hb ≤ 8 g/dL. Furthermore, several obstetric societies, including The Royal College of Obstetricians and Gynecologists (UK) and the French College of Gynecologists and Obstetricians, have published PPH guidelines recommending that a Hb level above 8 g/dL is a therapeutic goal.41 These observations, coupled with our findings, have important clinical relevance for several reasons. First, antenatal anemia is a modifiable risk factor for transfusion. Second, in the nonobstetric literature, limiting anemia before surgery is a key component of patient blood management.42 With antenatal anemia affecting up to 25% pregnant women,43 similar initiatives may be necessary to promote anemia correction before delivery. Future studies are needed to examine whether optimization of maternal Hb levels before delivery limits the likelihood of postpartum RBC transfusion. Third, RBC transfusion may be used during acute episodes of blood loss as well as for treating symptomatic postpartum anemia in a nonbleeding patient. Studies are also needed to determine best approaches for anemia management during and after an episode of severe PPH.
We identified a number of other well-established risk factors, including multiple gestations (both cohorts), placenta previa (prelabor CD), and chorioamnionitis (intrapartum CD).5,15,31,44 Other risk factors are less recognized. Hispanic women had a 2-fold increased odds of severe PPH compared with whites, after prelabor CD, but not intrapartum CD. Disparities in PPH have been described by others.45 These may due to differences in the provision of medical care as well as underlying social, biological, and genetic factors. Only 1 study has investigated whether the type of uterine incision influences PPH risk. Combs et al21 reported that a classical uterine incision was an independent risk factor. Compared with a low transverse incision, a greater degree of blood loss may occur with nontransverse incisions because thicker and more vascular myometrial tissue may be surgically incised. The risk of PPH among women with earlier D&E or D&C may be related to altered placentation.46 Future studies are needed to determine whether this association is observed among women undergoing intrapartum CD. The relationship between GDM and severe PPH may be explained by fetal macrosomia, a recognized risk factor for PPH.47 In the intrapartum CD cohort, women with a history of 1 previous CD had a lower adjusted odds of severe PPH compared with women with no previous CDs. Because of concern of uterine rupture, it is possible that obstetric providers are less likely to consider long periods of labor or augmentation for women undergoing trial of labor after previous CD. This may partly explain why these women were at lower risk of severe PPH compared with those without a history of previous CD.
Our study had a number of limitations. Although we considered a large number of candidate variables, unmeasured factors may have influenced PPH risk, such as obstetric or anesthesia provider experience. In a systematic review, Bauer et al48 previously reported that the risk of failed conversion of labor epidural analgesia to CD anesthesia is higher if care is provided by a nonobstetric anesthesiologist compared with care given by an obstetric anesthesiologist. Studies are needed to evaluate whether specific anesthetic practices that may influence the magnitude of blood loss, such as the use of high concentrations of volatile anesthetic and blood product use, vary according to anesthesia provider experience. Because of the low frequency of several factors, such as fibroids and prolonged second-stage labor, we could not accurately examine associations between these variables with severe PPH. We did not collect data for the frequency, timing, or indications for patients who underwent intraoperative conversion from neuraxial to general anesthesia. Given that combined rates of preoperative and intraoperative conversion from neuraxial to general anesthesia are low (1.7%–5.8%),49–51 any influence of intraoperative conversion on the point estimates for severe PPH among those undergoing neuraxial anesthesia is likely to be small.
We did not assess risk factors among women who underwent elective or nonelective CD because we chose to differentiate our cohorts by the presence versus the absence of spontaneous labor or induction of labor. We chose this approach because patient characteristics and CD indications are known to markedly differ across these cohorts.32 Furthermore, determining the “electiveness” of CD may have been more challenging, resulting in misclassification bias.
We identified women with abnormal placentation based on information in the medical record documented after the time of delivery. We elected not to include abnormal placentation as an a priori risk factor because we could not ascertain, from chart review, whether all cases of abnormal placentation were suspected during the antenatal period. We acknowledge this as an important limitation because abnormal placentation is strongly associated with major postpartum hemorrhage and massive transfusion.52 However, placenta previa and number of earlier CDs are well-known risk factors for abnormal placentation,53 and both variables were considered as candidate variables in our regression model for the prelabor cesarean cohort.
The findings from our analyses cannot be used to determine individual patient prediction of severe PPH. Such analyses would require large population-wide cohort studies using nuanced clinical data. However, our data could be used to guide investigators in the selection of relevant candidate variables in studies investigating prediction models of PPH during CD. Relevant prediction models require external validation in independent cohorts.
In conclusion, our findings suggest that the risk factor profile for severe PPH differs according to whether CD is performed before or after the onset of labor or induction. To confirm these findings, population-wide studies are needed to determine whether data should be partitioned according to CD subtype.
Criteria for Arrest of First Stage of Labor and Prolonged Second Stage of Labor
We defined arrest of active phase of the first stage of labor if latent phase was completed and no documented evidence of cervical change for >2 hours. These criteria were also used to define prolonged second stage for nulliparous women (second stage exceeding 2 hours and 3 hours with and without regional anesthesia, respectively) and for multiparous women (second stage exceeding 1 hour and 2 hours with and without regional anesthesia, respectively).
These definitions were based on guidelines published by the American College of Obstetricians and Gynecologists in 2003.25
The authors are grateful for the assistance of Dr G. Hilton and L. Tian who assisted with some of the data collection for this project. They also wish to acknowledge the support of researchers based in the Stanford Center for Clinical Informatics (Stanford School of Medicine) for identifying patients study from STRIDE and electronic hospital databases. STRIDE is a research and development project at Stanford University to create a standards-based informatics platform supporting clinical and translational research. The STRIDE project is supported by the National Center for Research Resources and the National Center for Advancing Translational Sciences, National Institutes of Health, through grant UL1 RR025744.
Name: Alexander J. Butwick, MBBS, FRCA, MS.
Contribution: This author helped conceive the work, acquire, analyze and interpret the data, and draft and revise the manuscript.
Name: Bharathi Ramachandran, BS.
Contribution: This author helped acquire the data.
Name: Priya Hegde, BA.
Contribution: This author helped acquire the data.
Name: Edward T. Riley, MD.
Contribution: This author helped conceive the work, interpret the data, and revise the manuscript.
Name: Yasser Y. El-Sayed, MD.
Contribution: This author helped conceive the work, interpret the data, and revise the manuscript.
Name: Lorene M. Nelson, MS, PhD.
Contribution: This author helped conceive the work, analyze and interpret the data, and revise the manuscript.
This manuscript was handled by: Jill M. Mhyre, MD.
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