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Previous Cesarean Delivery and Risks of Placenta Previa and Placental Abruption

From the ¹ Division of Epidemiology and Biostatistics and ² Division of Maternal–Fetal Medicine, Department of Obstetrics, Gynecology, and Reproductive Sciences, Robert Wood Johnson Medical School, University of Medicine and Dentistry of New Jersey, New Brunswick, New Jersey.

	ABSTRACT

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OBJECTIVE: To examine the association between cesarean delivery and previa and abruption in subsequent pregnancies.

METHODS: A retrospective cohort study of first 2 (n = 156,475) and first 3 (n = 31,102) consecutive singleton pregnancies using the 1989–1997 Missouri longitudinally linked data were performed. Relative risk (RR) was used to quantify the associations between cesarean delivery and risks of previa and abruption in subsequent pregnancies, after adjusting for several confounders.

RESULTS: Rates of previa and abruption were 4.4 (n = 694) and 7.9 (n = 1,243) per 1,000 births, respectively. The pregnancy after a cesarean delivery was associated with increased risk of previa (0.63%) compared with a vaginal delivery (0.38%, RR 1.5, 95% confidence interval [CI] 1.3–1.8). Cesarean delivery in the first and second births conferred a two-fold increased risk of previa in the third pregnancy (RR 2.0, 95% CI 1.3–3.0) compared with first two vaginal deliveries. Women with a cesarean first birth were more likely to have an abruption in the second pregnancy (0.95%) compared with women who had a vaginal first birth (0.74%, RR 1.3, 95% CI 1.2–1.5). Two consecutive cesarean deliveries were associated with a 30% increased risk of abruption in the third pregnancy (RR 1.3, 95% CI 1.0–1.8). A second pregnancy within a year after a cesarean delivery was associated with increased risks of previa (RR 1.7, 95% CI 0.9–3.1) and abruption (RR 1.5, 95% CI 1.1–2.3).

CONCLUSION: A cesarean first birth is associated with increased risks of previa and abruption in the second pregnancy. There is a dose–response pattern in the risk of previa, with increasing number of prior cesarean deliveries. A short interpregnancy interval is associated with increased risks of previa and abruption.

LEVEL OF EVIDENCE: II-2

Surgical disruption of the uterine cavity is a potential risk factor for placenta previa and placental abruption.^9,10 Cesarean delivery is the most common of operative procedures in the United States, accounting for well over a fourth of all deliveries.^11,12 It is known to cause lasting damage to the myometrium and endometrium.¹³ The first observation that reported an association between prior cesarean delivery and increased risk of placenta previa dates back to the early 1950s.¹⁴ Several studies have since corroborated the association both for placenta previa and placental abruption.^9,10,17 These findings were subsequently confirmed through a large meta-analysis of more than 3.7 million pregnant women.¹⁵ The fetus in a pregnancy after a cesarean delivery or other uterine surgical procedure may be at higher risk of morbidity and mortality than one in a pregnancy in which there was no preceding cesarean delivery. The increased risk of morbidity and mortality may partly be due to the relative increase in abruption and previa to pregnant mothers with previous (obstetric) surgical procedures.^18,19 However, it remains unclear as to whether these risks increase with the number of cesarean deliveries in a dose-dependent fashion. For instance, whether patients who have undergone a single cesarean delivery run a similar risk of previa and abruption as those patients who have undergone two or more prior cesarean deliveries remains unexplored. This information is important from the point of view of assigning patients in terms of risk profiles and for counseling.

Studies have reported that both short and long interpregnancy intervals are associated with an array of adverse pregnancy outcomes. Increased risk of abnormally adherent placentas has also been reported among pregnancies with short interpregnancy intervals.²⁴ Interpregnancy interval–specific risks of placenta previa and placental abruption among women with previous cesarean deliveries have not been examined; therefore, little is known regarding their associations.

To address these issues, we undertook this study with the following objectives: 1) to estimate if a cesarean delivery is a risk factor for placenta previa and placental abruption in subsequent pregnancies; 2) to examine for the presence of a dose–response relationship between prior cesarean deliveries and risks of previa and abruption in subsequent pregnancies; and 3) to examine if the risk of previa and abruption in relation to prior cesarean delivery is modified by the interval between pregnancies.

	MATERIALS AND METHODS

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Data for this study were obtained from the Missouri longitudinally linked birth certificate and fetal mortality data files for the years 1989 to 1997, inclusive. In this dataset, siblings are linked to their biological mothers using unique identifiers. The methods and algorithm used in linking birth certificate data into sibships and the process of validation have been described in detail previously.²⁵ The Missouri vital record system is considered very reliable and one that has been adopted as a "gold standard" to validate U.S. national datasets that involve matching and linking procedures.²⁶ The linked data essentially contains information on both live birth and fetal death for each sibling and provides a platform for a longitudinal study of birth outcomes for each pregnancy. The database comprised 706,075 live births and fetal and infant deaths for which records were available between 1989 and 1997. Information on live birth and fetal and infant death, as well as maternal sociodemographic and behavioral characteristics, medical history, and complications during labor and delivery were included in the data files.

Using an analytic cohort of linked first two singleton births in the state of Missouri from 1989 to 1997 (n = 157,831) and the first three consecutive singleton births (n = 31,699), we conducted an analysis to examine the association between previous cesarean delivery and risks of placenta previa and placental abruption in subsequent singleton pregnancies, and if a short interpregnancy interval increases the risk.

The study was approved by the ethics committee of the Institutional Review Board of the University of Medicine and Dentistry of New Jersey–Robert Wood Johnson Medical School, New Brunswick, NJ. Statistical analysis was performed using SAS 9.1 (SAS Institute, Cary, NC).

Data on maternal characteristics were based on the study cohort that comprised the first two births. Self-reported maternal race was grouped as white, black, Hispanic, and others races. Factors that were considered potential confounders included maternal age (< 20, 20–24, 25–29, 30–34, 35 years), marital status (married/unmarried), late initiation of prenatal care (after first trimester), maternal education (< 12, 12, and 13 years of completed schooling), maternal smoking and alcohol use during pregnancy (yes/no), and interval between a birth and a subsequent pregnancy lasting at least 20 weeks (interpregnancy intervals of < 1, 1.0–1.5, 1.5–2.0, 4 years). The outcomes that were examined included risks of placenta previa and placental abruption.

We first examined the distributions of maternal sociodemographic and behavioral characteristics by previous cesarean delivery. To examine the association between abruption and previa in subsequent pregnancies (first two and first three pregnancies) a logistic regression model was fitted after controlling for potential confounding variables (maternal age, maternal race, maternal education, prenatal care, marital status, interpregnancy interval, and smoking and alcohol use during pregnancy). Relative risks (RRs) and 95% confidence intervals (CIs) were used to quantify the association. Because the incidence of the outcome was fairly low in our study, odds ratios derived from the logistic regression models were interpreted as RRs. Potential confounding variables were either chosen a priori or were factors that resulted in a shift of at least 10% between the unadjusted and adjusted RRs.

From a total of 711,015 births in the state of Missouri between 1989 and 1997, we excluded the following categories: multiple births (n = 19,969), pregnancies that ended at less than 20 weeks of gestation and fetuses that weighed less than 500 g (n = 25,850), and births with missing method of delivery (n = 72).

	RESULTS

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Among the 156,475 women with their first two pregnancies, the overall incidence of previa and abruption were 4.4 and 7.9 per 1,000 singleton births, respectively. The sociodemographic and behavioral characteristics of 39,661 (25% of 156,475) mothers with a history of cesarean are presented in Table 1. Women with a cesarean delivery were more likely to be white, married, of advanced maternal age, to have completed 12 years or more of schooling, and to have initiated prenatal care early in the pregnancy.

Table 2 outlines the risks of previa by previous cesarean delivery histories. Risk for previa in the second birth was 50% higher among women with a prior cesarean delivery (RR 1.5, 95% CI 1.3–1.8). A cesarean first birth and vaginal second birth and vice versa did not increase the risk of previa in the third birth. However, the risk of previa was two-fold higher (RR 2.0, 95% CI 1.3–3.0) in women with cesarean first and second births, compared with women with vaginal deliveries in the first and second births.

Table 3 shows the risks of abruption by previous cesarean delivery histories. The risk of abruption in the second birth was 30% higher when first birth was cesarean (RR 1.3, 95% CI 1.2–1.5). Regardless of the mode of delivery of the first birth, a cesarean second birth was associated with increased risk of abruption. A vaginal first birth followed by a cesarean birth conferred a nonsignificant increase in the risk of abruption (RR 1.5, 95% CI 0.9–2.2), whereas a cesarean first birth followed by a cesarean second birth conferred a marginally increased risk of abruption (RR 1.3, 95% CI 1.0–1.8). The risks of abruption among cesarean first births and vaginal second births were, however, similar.

A second pregnancy within a year after the first was not associated with an increased risk of previa among women whose first birth was vaginal, but the risk was increased by 70% in women who had a cesarean first birth (RR 1.7, 95% CI 0.9–3.1). Although there is clear evidence that the risk of previa among women with or without previous cesarean deliveries decreases between the first and second years of interpregnancy interval, the risk in second pregnancy among women with prior cesarean delivery remained higher than the risk in the first pregnancy (Fig. 1).

View larger version (13K): [in this window] [in a new window]   Fig. 1. Risk (A) and adjusted RR (B) of placenta previa by previous cesarean delivery and interpregnancy interval: Missouri, 1989–1997. Open circles correspond to no previous cesarean; solid circles correspond to women with previous cesarean. Getahun. Prior Cesarean and Risks of Abruption and Previa. Obstet Gynecol 2006.

Irrespective of the method of delivery in the first birth, a second pregnancy within the first year postpartum increased the risk of abruption. A second pregnancy within the first year postpartum increased the risk of abruption by 52% in a woman with a vaginal first birth and by 111% in a woman with a cesarean first birth. Women with a cesarean first birth were more likely to have an abruption if the second pregnancy occurred within a year after the first compared with women with vaginal first birth (RR 1.5, 95% CI 1.1–2.3). A decreasing risk of abruption in the second pregnancy was noted among women with and without previous cesarean in every interpregnancy interval between the first and the second years. Although the risk of abruption in women with previous vaginal birth is lower than risk in the first pregnancy, when the second pregnancy is delayed by at least 1.5 years, the risk for those with previous cesarean deliveries remained unchanged from the risk in the first pregnancy (Fig. 2).

View larger version (13K): [in this window] [in a new window]   Fig. 2. Risk (A) and adjusted RR (B) of placental abruption by previous cesarean delivery and interpregnancy interval: Missouri, 1989–1997. Open circles correspond to no previous cesarean; solid circles correspond to women with previous cesarean. Getahun. Prior Cesarean and Risks of Abruption and Previa. Obstet Gynecol 2006.

	DISCUSSION

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Many studies that have examined risks of uteroplacental bleeding disorders (placenta previa and placental abruption) have identified potential risk factors including maternal age, race, marital status, parity, prenatal care, cocaine use, and smoking during pregnancy.^1–8,15 The association between prior cesarean delivery and risks of previa and abruption are, however, limited. With the increasing trend in cesarean deliveries observed in recent years in the United States,^11,12 the incidence of previa and abruption is expected to rise. In the present study, a cesarean first birth was associated with an increased risk of placenta previa and placental abruption in subsequent pregnancies. This observation concurs with those of Hemminki and Merilainen¹⁰ who reported that a cesarean increased the risk of abruption in a subsequent pregnancy. Similarly, Lydon-Rochelle et al¹⁷ found an increased risk of abruption in the second pregnancy among women with a prior cesarean. These authors used the linked birth certificate and hospital discharge data to examine the association between prior cesarean delivery and abruption, but they did not examine the impact of a short interpregnancy interval and risk of previa and abruption. Our study extends these observations to women in their first three pregnancies. We observed a 30–50% increase in the risk of abruption in third birth among women with a cesarean second birth, irrespective of the method of delivery in their first birth. To examine the possibility of bias due to recurrence of abruption in subsequent pregnancies, we repeated the analysis after excluding observations with abruption in first and second births. The magnitude of the estimates (shown in Table 3) remained unchanged.

Using the 1984–1987 Washington state birth certificate data, Taylor et al¹⁶ found in their case-control study an increased risk of previa in women with previous cesarean delivery (odds ratio 1.48, 95% CI 1.13–1.95). However, because they used birth certificate data that was not linked to subsequent pregnancies, they were not able to examine risks between two consecutive pregnancies. Norwegian investigators reported a 32% increase in the risk of placenta previa in the second pregnancy in women when the first birth was cesarean.²⁷ We demonstrated a 50% increase risk in previa in the second birth when the first birth was cesarean. Furthermore, we showed that this risk was doubled in the third birth when the first birth and second birth were both cesarean, suggesting a dose–response risk gradation. This finding suggests that encouraging women to deliver vaginally may be an important step in lowering risk of previa and abruption.

Pathological changes in the myometrium and endometrium of the uterus have been described in the presence of previous cesarean delivery scar. These include polyp formation, lymphocyte infiltration, capillary dilatation, and infiltration of the endometrial tissue that surround the scar by free red blood cells.¹³ These observations suggest that the pathological changes in the vicinity of cesarean delivery scars may create suboptimal implantation of the placenta, increased vascular malformations, and increased fragility of vessels that are known risk factors for abruption. Furthermore, rupture of the spiral arteries may lead to the formation of decidual hematomas, which may likely culminate in placental abruption. The same pathological changes of the endometrium and uterine cavity may be responsible for the increased risk of placenta previa among women with prior cesarean delivery.

Previous studies have reported that both short and long interpregnancy intervals are associated with adverse pregnancy outcomes including stillbirth, preterm birth, small-for-gestational-age birth, and neonatal mortality. Wax et al²⁴ reported increased risk of abnormally adherent placentas (placenta accreta, increta, and percreta) among pregnancies with short interpregnancy intervals. Our finding of increased risk of placenta previa in women with cesarean first delivery and increased risk of abruption in pregnancy conceived within 2 years point toward the benefit of postponing pregnancies by at least 2 years.

Median interpregnancy interval for the second pregnancy among women with first vaginal birth and first cesarean birth were 2.2 and 2.4, respectively. But method of deliveries–specific interpregnancy intervals was lower for the third pregnancy as compared with second pregnancy. Although first two vaginal births, regardless of the interpregnancy intervals, confer no risk, first two cesarean births concurrently with low interpregnancy interval is a significant risk factor for previa and abruption and deserve special attention in counseling.

The pathophysiologic conditions as to why a short interpregnancy interval increases risk of adverse pregnancy outcome is not fully understood. However, this is probably explained by the maternal depletion theory.^28,29 Pregnancy is a physiologically demanding condition to the mother that may lead to depletion of stored nutritional elements. A pregnancy with a short (interpregnancy) interval may deprive the mother from restoring those nutritional elements needed to support a normal pregnancy, full recovery of the internal lining of the uterus.

Wilcox and Gladen³⁰ proposed that investigators studying successive pregnancies be aware of the role of selective fertility, a phenomenon likely to occur when couples attempt to replace a pregnancy loss more quickly than those with normal outcomes. This has implications to our study owing to the high rates of perinatal mortality associated with placental abruption^6,8 and placenta previa,³¹ and the desire to achieve a completed family size quickly.^30,32 If selective fertility were indeed likely to operate, it can be speculated that the interpregnancy interval is shorter in such couples.

To address this concern, we carried out a separate analysis after limiting the study to all live births that survived to infancy. The pattern of associations was essentially unchanged (not shown) from those of our original analysis (Tables 2 and 3), lending further credibility and robustness to our findings. One plausible reason for this is that our original analysis was already restricted to women with only the first two and first three pregnancies (ie, there was a smaller "exposure" window). Second, acting on the speculation that this approach will not completely address the bias due to selective fertility (for example, a live birth that was associated with a serious pregnancy complication may have forced the woman to decline an attempt at vaginal birth in a subsequent pregnancy), we repeated the analysis after excluding premature rupture of membranes, eclampsia, maternal fever, and excessive bleeding during delivery—factors necessitating a cesarean delivery. The magnitude of the estimates remained unchanged, suggesting that the effects of selective fertility, if present, are likely minimal.

Other biases and limitations of our study are those typical of population-based studies that rely on vital statistics data. The birth certificate data are prone to some degree of under-reporting of certain variables (eg, smoking during pregnancy, medical and obstetric risk factors), which could introduce systemic or random bias.^33,34 The vital statistics data are often collected after the termination of the pregnancy, thereby introducing a misclassification of certain risk factors (such as smoking) in pregnancy. This misclassification is likely to be differential in nature, and if present, will bias the effect measures away from the null.³⁵ The possibility of our results being affected by residual confounding due to unmeasured factors (such as cocaine use) may have also affected the associations noted here. Conversely, the strengths of this study include the large population-based study cohort and our controlling for a variety of potential confounding variables.

A cesarean birth is an important risk factor for placenta previa and placental abruption in a subsequent pregnancy. The presence of a dose-response pattern in the risk of placenta previa with increasing number of prior cesarean deliveries, coupled with a biologically plausible association, provides compelling evidence in support of the association. Irrespective of the method of delivery in the first pregnancy, a short interpregnancy interval appears to be associated with increased risks of both placenta previa and placental abruption. The effects of these associations on adverse pregnancy outcomes remain unknown.

	Footnotes

 
See related editorial on page 752.

Drs. Getahun and Ananth are supported through a grant (R01-HD038902) from the National Institutes of Health, awarded to Dr. Ananth.

The authors thank the Missouri Health Department for graciously allowing us to utilize the state maternally linked longitudinal data file.

Corresponding author: Cande V. Ananth, PhD, MPH, Division of Epidemiology and Biostatistics, Department of Obstetrics, Gynecology, and Reproductive Sciences, UMDNJ-Robert Wood Johnson Medical School, 125 Paterson Street, New Brunswick NJ 08901-1977; e-mail: cande.ananth{at}umdnj.edu.

doi:10.1097/01.AOG.0000206182.63788.80

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This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Getahun, D. Articles by Ananth, C. V. Search for Related Content PubMed PubMed Citation Articles by Getahun, D. Articles by Ananth, C. V. Related Collections Labor and operative obstetrics Placenta