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ORIGINAL RESEARCH |
From the Department of Health Services, School of Public Health and Community Medicine; Department of Family and Child Nursing, School of Nursing; Department of Epidemiology, School of Public Health and Community Medicine; and Department of Obstetrics and Gynecology, School of Medicine, University of Washington, Seattle, Washington.
Address reprint requests to: Mona Lydon-Rochelle, PhD, MPH Center for Women’s Health Research Mailstop 357262 University of Washington Seattle, WA 98195-7262 E-mail: minot{at}u.washington.edu
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Abstract |
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Methods: We conducted a population-based, retrospective cohort analysis using data from the Washington State Birth Events Record Database. The study cohort included all primiparas who gave birth to live singleton infants in nonfederal short-stay hospitals from January 1, 1987, through December 31, 1996, and who had second singleton births during the same period (n = 96,975). Odds ratios (ORs) and 95% confidence intervals (CIs) were calculated for placental abruption or previa at second births associated with first-birth cesareans.
Results: Among our study cohort, abruptio placentae complicated 11.5 per 1000 and placenta previa 5.2 per 1000 singleton deliveries at second births. In logistic regression analyses adjusted for maternal age, women with first-birth cesareans had significantly increased risk of abruptio placentae (OR 1.3, 95% CI 1.1, 1.5), and placenta previa (OR 1.4, 95% CI 1.1, 1.6) at second births, compared with women with prior vaginal deliveries.
Conclusion: We found moderately increased risk of placental abruption and previa as a long-term effect of prior cesarean delivery on second births.
Cesarean delivery is the most common major operative procedure in the United States, at a rate of 21.2 per 100 deliveries in 1998.1,2 It has been associated with increased risk of placental abruption and previa in subsequent births, conditions resulting in increased likelihood of low birth weight (LBW), preterm delivery, and perinatal death.3–13
Given the commonness of cesarean delivery and significant consequences of placental abruption and previa, the associations between those events have considerable clinical and public health importance. However, the results of epidemiologic studies on the relationship between cesarean delivery and subsequent placental abruption or previa vary substantially across populations and by study design, and commonly have not accounted for the important confounding influences of reproductive factors such as prior placental abruption or previa, parity, and prior method of delivery. To explore this issue, we used statewide, maternally linked birth certificate and hospital discharge data to examine the associations between first-birth method of delivery and placental abruption and previa at second birth.
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Materials and Methods |
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Deliveries were classified as cesarean if "cesarean" was checked on the birth certificate, or any of International Classification of Diseases, 9th Revision, Clinical Modification (ICD-9-CM) cesarean delivery procedure or diagnosis codes were coded on hospital discharge data.15 Given those criteria, there were 19,875 (20.8%) women who delivered by cesarean and 75,755 (79.2%) women who delivered vaginally available for analysis.
An adverse placental outcome was classified as abruptio placentae if "placental abruption" was checked on the birth certificate, or ICD-9 diagnosis code 641.2 (premature separation of placenta) was coded on hospital discharge data. It was classified as placenta previa if "placenta previa" was checked on the birth certificate, or ICD diagnosis codes 641.0 (placenta previa without hemorrhage noted before labor and delivered by cesarean delivery) or 641.1 (hemorrhage from placenta previa) were coded on hospital discharge data.
To examine the risk for adverse placental outcomes at second birth associated with first-birth cesarean delivery, we used logistic regression to calculate odds ratios (ORs) and 95% confidence intervals (CIs), using women with first-birth vaginal deliveries as the referent group.16 In analyses of placental abruption and previa, interactions between first-birth method of delivery and major puerperal infection at first birth, year of delivery at second birth, and maternal age at second birth were assessed. To assess interactions, we used the likelihood ratio test, with P < .05 denoting statistical significance, and no significant interactions were found. The following variables, reported at second delivery, were examined for possible confounding effects in all analyses: maternal age, maternal race or ethnicity, interbirth interval, payer, history of induced abortion, history of spontaneous abortion, marital status, maternal smoking during pregnancy, onset of prenatal care after first trimester, infant birth weight, infant estimated gestational age, and infant gender. All variables were considered to be confounders if their inclusion changed any of the model’s ORs for second-birth abruptio placentae or placenta previa associated with first-birth method of delivery by 10% or more.17 Only maternal age met that criterion.
Placental abruption might be attributable to medical and pregnancy complications, so in one subanalysis we examined the risk of abruption among only the 92,433 (96.7%) women without chronic hypertension, moderate to severe preeclampsia, or diabetes mellitus identified at first or second births. We also were interested in the possible clinical importance of placental abruption or previa among women in our study because the severity of those outcomes cannot be determined from ICD-9 diagnostic codes. We examined frequency of selected complications among women with and without placental abruption or previa, including cesarean delivery, postpartum hemorrhage, severe posthemorrhagic anemia, blood transfusion, major puerperal infection, hysterectomy, preterm birth, LBW, fetal death, and infant death. Differences in complication rates for women with placental abruption or previa relative to women without either were compared by Mantel-Haenszel 
2 test.
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Results |
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. Except for maternal age, which was greater among women with first-birth cesarean deliveries than among women with vaginal deliveries, second-birth demographic and perinatal characteristics of the groups were similar. Abruptio placentae complicated 11.5 per 1000 singleton deliveries at second births. The abruption rate was higher among women with first-birth cesarean deliveries (13.7/1000) than women with first-birth vaginal deliveries (10.9/1000). In a logistic regression model adjusted for maternal age, women with first-birth cesarean deliveries had a 30% increased risk of placental abruption at second births compared with women with prior vaginal deliveries (OR 1.3, 95% CI 1.1, 1.5). We repeated this analysis among women without selected medical and pregnancy complications, with identical results.
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Maternal and perinatal complications at second births differed by adverse uteroplacental outcome status (Table 2). With the exceptions of fetal deaths among women with placental abruption and hysterectomies among women with placental previa, all differences in complication rates compared with no placental abruption or previa were significant (P < .001). Women with placental abruption or previa were more likely than women without either to have cesarean deliveries, postpartum hemorrhages, severe posthemorrhagic anemia, blood transfusions, and major puerperal infections. The infants of women with placental abruption or previa were more likely to be preterm (estimated gestational age under 36 weeks), LBW (under 2500 g) or die after delivery.
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Discussion |
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Although we found a statistically significant increased risk of subsequent abruption among women with prior cesarean deliveries, our observed risk was lower than reported in population-based European studies.8,11,13 A Finnish birth registry study reported an OR of 2.4, but a history of prior abruption was not considered.8 A Swiss study found a lower association (OR 1.5), but did not control for prior placental abruption, parity, or number of cesareans.13 Our results were most similar to a Norwegian birth registry–linked study that reported an OR of 1.4 among women of all parities.11 They adjusted for parity and prior cesarean delivery among women without histories of placental abruption. A uterine scar adjacent to an anterior placenta could cause impaired placental perfusion, so a plausible explanation is that the blood flow to the intervillous space is inadequate and might increase risk of abruption.11
The 40% increased risk of second-birth placenta previa we found associated with cesarean delivery also was lower than risks reported by previous studies, which range from 1.5 to 4.5.4–10,13 However, none of those studies accounted for prior placenta previa and some did not address influence of parity or number of prior cesarean deliveries.5,7,9,10,13 Such biases would likely overestimate any observed cesarean-associated risk because the risk of previa is known to increase with those reproductive factors.18–22 The uterine scarring associated with cesarean delivery has resulted in endometrial and myometrial damage, defective implantation mechanisms, and failure of differential growth of the scarred lower uterine segment, all of which predispose to low implantation of the placenta.4,8,9,23
The use of longitudinally linked birth certificate and hospital discharge data to investigate associations between first-birth delivery method and subsequent adverse placental outcomes has a number of advantages. Our dataset allowed us to examine an entire cohort of primiparas who gave birth to live singletons and their next consecutive singleton births over a 10-year period, thus providing an adequate number of subjects to examine with precision relatively uncommon outcomes. Our data linkage process also allowed us to measure and take into account the influence of important reproductive factors that have potential to distort associations between cesarean delivery and those outcomes, thus avoiding likely overestimation of cesarean-associated risk in prior studies.
Data derived from birth certificates and hospital discharge records might be limited in accuracy or completeness. In this study, we have some assurance that there was minimal misclassification of method of delivery because previous research reported that 99.8% of cesarean deliveries are correctly classified using Washington State–linked birth certificate-hospital discharge files.24 We were unable to identify grade of placental abruption or the clinical severity of placenta previa because the assigned ICD-9 diagnostic codes on hospital discharge data lacked specific clinical information. For example, it was possible that clinically mild placental abruption would be more accurately diagnosed among women with cesarean deliveries than among women with vaginal deliveries at second birth, which could lead to overestimation of risk associated with first-birth cesarean delivery.
Increased risk of placental abruption and previa might be attributed to factors other than prior cesarean delivery, and those factors also might influence delivery method. We attempted to isolate effects of delivery method in several ways. We restricted our analysis to women with second singleton births, eliminating potential confounding effects of parity and multiple gestations. We excluded from all analyses women with previous placental abruption or previa, which could have predisposed them to cesarean delivery and recurrence of adverse placental outcomes.25–28 Also, placental abruption might be attributable to high-risk medical or pregnancy conditions rather than method of delivery, so in one subanalysis we included only women without medical or pregnancy complications. We also were concerned that risk of adverse placental outcomes associated with cesarean delivery might be higher among women with prior major puerperal infections; however, lack of interaction argues against that interpretation.
In addition to increasing risk for preterm birth, LBW, and perinatal mortality, placental abruption or previa carry important maternal health consequences, including obstetric hemorrhage, severe posthemorrhagic anemia, and major puerperal infection. Our study linked population-based pregnancy data to measure and account for important confounding factors. We found only a moderately increased risk of placental abruption and previa as long-term effects of prior cesarean delivery on women at second births. Given the slight increase in risk for those subsequent outcomes among women at second births, our results provide new information that clinicians might consider when counseling women during pregnancy.
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Footnotes |
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We appreciate the data linkage programming support of William O’Brien.
Received October 11, 2000. Received in revised form December 11, 2000. Accepted January 12, 2001.
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References |
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2. Ventura SJ, Martin JA, Curtin SC, Matthews TJ, Park MM. Births: Final data for 1998. National Vital Statistics Reports 2000;48(3):13.
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24. Parrish KM, Holt VL, Connell FA, Williams B, LoGerfo JP. Variations in the accuracy of obstetric procedures and diagnoses on birth records in Washington State, 1989. Am J Epidemiol 1993;138: 119–27.
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28. Misra DP, Ananth CV. Risk factor profiles of placental abruption in first and second pregnancies: Heterogeneous etiologies. J Clin Epidemiol 1999;52:453–61.[Medline]
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