The Likelihood of Placenta Previa With GreaterNumber of Cesarean Deliveries and Higher Parity

Melissa Gilliam, MD, MPH, Deborah Rosenberg, PhD, and Faith Davis, PhD

OBJECTIVE: To examine the relationship between prior ce-sarean delivery and placenta previa.

METHODS: A hospital-based, case-control study was con-ducted in which 316 multiparous women with placentaprevia were identified. Controls consisted of 2051 multipa-rous women with spontaneous vaginal deliveries. Informa-tion on prior cesarean delivery was examined in threeforms: as a dichotomous variable, as an ordinal variable,and as a set of three indicator variables for one, two, andthree or more cesarean deliveries. Multivariable logisticregression modeling was used to obtain an adjusted esti-mate of this association.

RESULTS: Women with a prior cesarean delivery weremore likely to have a placenta previa than those without(odds ratio [OR] 1.59, 95% confidence interval [CI] 1.21,2.08). The likelihood of placenta previa increased as bothparity and number of cesarean deliveries increased. Thus,the adjusted OR for a primiparous woman with one cesar-ean delivery was 1.28 (95% CI 0.82, 1.99). For a woman whohas four or more deliveries with only a single cesareandelivery, the OR increases to 1.72 (95% CI 1.12, 2.64). Thistrend continues with greater parity and a greater numberof cesarean deliveries such that the likelihood of placentaprevia for a woman with parity greater than four andgreater than four cesarean deliveries was OR 8.76 (95% CI1.58, 48.53).

CONCLUSION: This study supports the association betweenprior cesarean delivery and placenta previa and demon-strates that the joint effect of parity and prior cesareandelivery is greater than that of either variable alone.(Obstet Gynecol 2002;99:97680. 2002 by the Ameri-can College of Obstetricians and Gynecologists.)

Placenta previa occurs when the placenta partially orcompletely occludes the internal cervical os. This condi-tion is a major cause of third-trimester bleeding, postpar-

tum hemorrhage, and maternal and neonatal morbidityand mortality. Placenta previa complicates approxi-mately 4.8 per 1000 deliveries in the United Statesannually, and placenta previa is fatal to the mother in0.03% of cases.1 Several risk factors for placenta previaexist including advanced maternal age, high parity,smoking, previous abortion, and prior cesarean deliv-ery.25

Controversy exists over the epidemiology of placentaprevia. Although many authors have commented on therelationship of placenta previa and cesarean delivery,68

the strength of this association remains in question.Studies of this relationship have not taken into accountboth the number of prior cesarean deliveries and the roleof potential confounding variables. If the likelihood ofplacenta previa increases with a greater number of cesar-ean deliveries, this finding would support the idea of acausal relationship between prior cesarean delivery andplacenta previa. Similarly, as the merits of elective repeatcesarean delivery are debated, possible consequences ofthis practice should be examined. The purpose of thisstudy is to more accurately estimate the likelihood ofplacenta previa after multiple cesarean deliveries and toexamine the effect of parity and other risk factors forplacenta previa on this association.

MATERIALS AND METHODS

A hospital-based, case-control study was conducted toexamine the relationship between prior cesarean deliv-ery and placenta previa. The Institutional ReviewBoards at the University of Illinois and Cook CountyHospital approved this study. Data were obtained fromtwo perinatal registries: the University of Illinois Perina-tal Center database and the Cook County HospitalPerinatal Center database. These databases include ev-ery delivery occurring at these institutions from 1986 to1989. Data for more than 450 maternal-fetal variablescontained in these databases were collected by trainedpersonnel through daily abstraction of medical records.Placenta previa was documented on the perinatal ab-stract form and entered as such in the perinatal registry if

From the Department of Obstetrics and Gynecology, and School of Public Health,University of Illinois at Chicago, Chicago, Illinois.

Presented in part at the 50th Annual Clinical Meeting of The American College ofObstetricians and Gynecologists, Chicago, Illinois, April 28May 2, 2001.

We thank Kristine Stolti, MPH, for statistical assistance and Arden Handler,DrPH, for consultation and advice.

976 VOL. 99, NO. 6, JUNE 2002 0029-7844/02/$22.00 2002 by The American College of Obstetricians and Gynecologists. Published by Elsevier Science Inc. PII S0029-7844(02)02002-1

there was mention of placenta previa in the medicalrecord. The abstracters codebook defined placenta pre-via as when the placenta lies partially or totally over thecervix rather than high in the uterus.3 The sample forthis study includes all multiparous women with singletonpregnancies and placenta previa identified in these data-bases during this time period, along with a 6% simplerandom sample of the remaining multiparous womenwith singleton pregnancies and spontaneous vaginal de-liveries. The final sample contained 316 cases and 2051controls.

Information on prior cesarean delivery was examinedin three forms: as a dichotomous variable history ofprevious cesarean delivery (yes, no), as an ordinalvariable number of previous cesarean deliveries, andas a set of three indicator (dummy) variables for one,two, and three or more previous cesarean deliveries,with no prior cesarean deliveries serving as the referencegroup. Information on potential confounders/effectmodifiers, including age, cigarette smoking during preg-nancy, induced abortion, spontaneous abortion, race/ethnicity, second-trimester bleeding, third-trimesterbleeding, and parity was also examined.

Univariate statistics for each variable, along with thebivariate association between each variable and placentaprevia were examined. Single-factor stratified analysiswas conducted to assess potential confounding or effectmodification of the association between prior cesareandelivery and placenta previa. Multivariable logistic re-gression modeling was then used to obtain an adjustedestimate of this association.

The multivariable regression modeling was carriedout separately for each of the three forms of the priorcesearean delivery variables. Importantly, interactionterms for parity and each of the three prior cesareandelivery variables were either statistically significant orclose to statistically significant when included in thesemodels (data not shown). This is not unexpected becausethe number of prior cesarean deliveries is by definitionconstrained by the total number of deliveries. To facili-tate interpretation of our results, therefore, the multiva-riable modeling was repeated, this time stratified byparity. The final models for each parity group includedage greater than 35, smoking, and prior abortion inaddition to the various forms of prior cesarean delivery.

Repeating the modeling three times permitted an as-sessment of overall association as well as an assessmentof trend. In particular, use of the ordinal variable fornumber of prior cesearean deliveries permits a test spe-cifically for linear trend, whereas use of the indicatorvariables permits an examination of trend with no as-sumption about its shape. Results for the ordinal variableare reported for zero, one, two, three, and four or more

prior cesarean deliveries, with corresponding parity cat-egories of one, two, three, and four or more. Results forthe indicator variables are reported for zero, one, two,and three or more prior cesarean deliveries, with corre-sponding parity categories of one, two, three or more.Fewer categories for the indicator variables were used toensure adequate sample size in each category becausevery few women had four or more prior cesarean deliv-eries.

RESULTS

Cases and controls did not differ significantly by race orby alcohol use, but cases were more likely to be of olderage, to have smoked cigarettes, and to have had a previ-ous abortion. In addition, in the current pregnancy, caseswere more likely to have obstetric complications includ-ing preterm delivery, second-trimester bleeding, andthird-trimester bleeding (Table 1).

Using the dichotomous variable any versus noprior cesarean delivery, the crude odds ratio (OR) for theassociation with placenta previa was 1.59 (95% confi-dence interval [CI] 1.21, 2.08, Table 2). Table 3 showsthis association stratified by parity and adjusting for agegreater than or equal to 35, smoking, and prior abortion.The relationship between placenta previa and cesareandelivery for the dichotomous variable was significant atparity two, three, and four or greater, and increased witheach level of parity.

Using the ordinal variable number of previous cesar-ean deliveries, the crude OR for the association withplacenta previa was 1.50 (95% CI 1.28, 1.77), indicatingan increase in the odds of placenta previa after cesareandelivery. Table 4 shows this association after stratifyingby parity and after adjustment. Here, the likelihood ofplacenta previa is significant and increasing as parityincreases. In addition, the likelihood of placenta previa issignificant and increasing as the number of prior cesar-ean deliveries increases within each parity group. Forexample, the adjusted OR for women of parity of four ormore who had three prior cesarean deliveries was 5.09(95% CI 1.41, 18.39). This value is greater than the ORsshown in Table 4 for women of the same parity who hadfewer prior cesarean deliveries, and also greater than theORs for women of lesser parity who had the samenumber of prior cesarean deliveries.

An increasing likelihood of placenta previa with in-creasing number of cesarean deliveries was also demon-strated using the indicator variables for one, two, andthree or more cesarean deliveries. The crude OR wasnot significant for one cesarean delivery compared withno cesarean deliveries, but the crude ORs for two cesar-ean deliveries compared with no cesarean deliveries and

977VOL. 99, NO. 6, JUNE 2002 Gilliam et al Cesarean Delivery and Placenta Previa

for three deliveries compared with no deliveries werestatistically significant (Table 2). Table 5 shows theadjusted OR for the association between prior cesareandelivery and placenta previa using the three indicatorvariables, stratified by parity. Again, the likelihood ofplacenta previa increased with a greater number of ce-sarean deliveries and also as parity increased. The ad-

justed ORs ranged from 1.28 (95% CI 0.82, 1.99) forprimiparous women to 4.05 (95% CI 1.63, 10.10) forwomen of parity of three or greater with three or moreprior cesarean deliveries. Thus, when using the indicatorvariables, the joint effect of parity and prior cesareandelivery also appears to be greater than the effect ofeither factor alone.

DISCUSSION

A number of studies have examined the contribution ofcesarean delivery to the risk of placenta previa in futurepregnancies. Yet, these studies failed to adjust for knownconfounding variables or to quantify the increased riskassociated with each additional cesarean delivery. Forexample, To and Leung contrasted the risk from a singleprior cesarean delivery with the risk of greater than oneprior cesarean delivery and showed an increased risk ofplacenta previa with more than one cesarean delivery.However, these authors did not control for confoundingvariables with multivariable analysis.7 Similarly, Clark etal showed that in women with one uterine incision therisk of placenta previa was 0.26% compared with 10% inwomen with four or more uterine incisions; however,this descriptive study did not control for known riskfactors for placenta previa.8 Finally, McMahon et alattempted to account for risk factors for placenta previaincluding age, race, parity, prior abortion, and smoking,but did not examine the role of multiple cesarean deliv-eries.6

In this study, by using a large and detailed database,we examined not only the relationship between onecesarean delivery and subsequent placenta previa, butalso the contribution of each additional cesarean deliveryto the development of placenta previa. In addition, the

Table 1. Demographic Factors, Lifestyle Variables, andObstetric History

Cases(n 316)

n (%)

Controls(n 2051)*

n (%)

Race or ethnicityWhite 136 (43.04) 958 (46.73)Black 66 (20.89) 401 (19.56)Hispanic 96 (30.38) 592 (28.88)Other 18 (5.7) 99 (4.83)

Age (y)

1519 7 (2.22) 116 (5.66)2029 168 (53.16) 1121 (54.66)3034 84 (26.58) 573 (27.94)35 57 (18.04) 241 (11.75)

Parity

1 125 (39.56) 1085 (52.9)2 101 (31.96) 598 (29.16)3 48 (15.19) 229 (11.17)4 42 (13.29) 139 (6.78)

Previous cesareandelivery

0 225 (72.12) 1643 (80.42)1 49 (15.71) 302 (14.78)2 29 (9.29) 82 (4.01)3 7 (2.24) 14 (0.69)4 2 (0.64) 2 (0.10)

Prior induced abortion

Yes 65 (20.57) 229 (11.17)No 251 (79.43) 1822 (88.83)

Prior spontaneousabortion

Yes 86 (27.22) 377 (18.38)No 230 (72.78) 1674 (81.62)

Smoking

Yes 95 (31.25) 431 (21.66)No 209 (68.75) 1559 (78.34)

Bleeding 28 wk

Yes 50 (15.82) 67 (3.27)No 266 (84.18) 1983 (96.73)

Bleeding 28 wk

Yes 85 (26.9) 19 (0.93)No 231 (73.1) 2031 (99.07)

Preterm

Yes 149 (47.3) 188 (9.19)No 166 (52.7) 1858 (90.81)

AlcoholYes 8 (2.56) 45 (2.22)No 304 (97.44) 1979 (97.78)

* Numbers for cases and controls may not always sum to the totalbecause of the presence of missing values.

Statistically significant at P .05.

Table 2. The Association Between Prior Cesarean Deliveryand Placenta Previa (Crude Odds Ratios and 95%Confidence Intervals)

Crude association for priorcesarean delivery (n 2355)*

Dichotomous variable for number ofprior cesarean deliveries: any vs none

1.591.21, 2.08

Ordinal variable for number of priorcesarean deliveries: 04

1.501.28, 1.77

Three indicator variables for priorcesarean delivery:

1 vs 0 1.180.84, 1.64

2 vs 0 2.561.64, 4.00

3 vs 0 3.621.45, 9.10

* The total sample size differs from that shown in Table 1 because ofthe presence of missing values.

978 Gilliam et al Cesarean Delivery and Placenta Previa OBSTETRICS & GYNECOLOGY

independent nature of this relationship was determinedby adjusting for multiple known risk factors for placentaprevia including age, smoking, and abortion. Using thismethodology, we demonstrated the importance of thenumber of cesarean deliveries when assessing the likeli-hood of future placenta previa. When we calculated thelikelihood of placenta previa using only a dichotomousvariable, any history of cesarean delivery, the OR was1.59. Once we considered the actual number of cesareandeliveries, the strength of this association became appar-ent: the OR for the likelihood of placenta previa withthree or more prior cesarean deliveries was 3.62.

In this study, we also demonstrated that the likelihoodof placenta previa increases with greater parity indepen-dent of the number of prior cesarean deliveries. In otherwords, the association between cesarean delivery andplacenta previa grows stronger as parity increases even ifthe number of cesarean deliveries stays the same. Wedemonstrated this relationship in two ways. First, withthe use of an ordinal variable (Table 4) in logistic regres-sion modeling we showed that the likelihood of placentaprevia increases with each additional pregnancy. Sec-ond, because use of an ordinal variable imposes a linearrelationship even when one does not truly exist, we alsoanalyzed the data using indicator variables (Table 5) for

the number of prior cesarean deliveries. The resultingORs, using the indicator variables, reflect the actualexperience of the women in our sample. Even using thisassumption-free approach, a similar pattern for the asso-ciation between prior cesarean delivery and placentaprevia emerged, although it was somewhat less pro-nounced than when the ordinal variable was used.Again, the likelihood of placenta previa increased bothacross and within parity groups.

The use of database records raises questions as to thequality of the data, as review of all medical records wasnot possible. For example, chart abstractors might havemissed cases of placenta previa even if this diagnosis wasrecorded in the medical record. Or, the medical recorditself might have been in error, if bleeding occurred whilein labor and a specific diagnosis of placenta previa wasnot made. However, there is no reason to believe thatmissed placenta previa cases were systematically differ-ent than the cases included in our study with regard tohistory of prior cesarean delivery. Change in diagnosis atthe time of cesarean delivery might also occur, but as theabstractions were performed on postpartum charts, suchchanges in diagnosis would have been apparent at thetime of abstraction.

One question that is raised and not answered in this

Table 3. The Association Between Prior Cesarean Delivery and Placenta Previa Based on a Dichotomous Variable for PriorCesarean Delivery (Adjusted Odds Ratios and 95% Confidence Intervals* Stratified by Parity)

Adjusted associationfor prior cesarean

delivery

Parity

1(n 1171)

2(n 675)

3(n 265)

4(n 171)

Any vs none 1.276 1.67 2.85 2.700.82, 1.99 1.02, 2.73 1.35, 6.02 1.09, 6.67

* Age 35 vs other, smoker vs nonsmoker, and any prior induced abortion are included in the adjusted model. The sample sizes for each parity level differ from those shown in Table 1 because of the presence of missing values.

Table 4. The Association Between the Number of Cesarean Deliveries and Placenta Previa Based on an Ordinal Variablefor Number of Prior Cesarean Deliveries (Adjusted Odds Ratios and 95% Confidence Intervals* Stratified byParity)

Number of priorcesarean deliveries

Parity

1(n 1171)

2(n 675)

3(n 265)

4(n 171)

0 1.0 1.0 1.0 1.01 1.28 1.40 1.60 1.72

0.82, 1.99 1.06, 1.84 1.15, 2.22 1.12, 2.642 1.95 2.56 2.96

1.13, 3.39 1.33, 4.93 1.26, 6.973 4.09 5.09

1.53, 10.96 1.41, 18.394 8.76

1.58, 48.53* Age 35 vs other, smoker vs nonsmoker, and any prior induced abortion are included in the adjusted model.

The sample sizes for each parity level differ from those shown in Table 1 because of the presence of missing values.

979VOL. 99, NO. 6, JUNE 2002 Gilliam et al Cesarean Delivery and Placenta Previa

study, or previous studies, is the role of prior placentaprevia as a risk factor for repeat placenta previa. Womenwho have a history of placenta previa have an increasedrisk of placenta previa in a subsequent pregnancy.9 In-formation on the indication for previous cesarean deliv-ery in our cases could not be obtained through review ofmedical records. Some women with prior cesarean deliv-ery may actually be cases of placenta previa. We, there-fore, raise this issue as a potential source of bias in thisstudy.

This study supports the conclusions of previous stud-ies showing an increased likelihood of placenta previa inwomen with prior cesarean delivery, and also shows therelationship of parity in the occurrence of placenta pre-via. Although one cesarean delivery does not signifi-cantly increase the likelihood of placenta previa in aprimiparous woman, subsequent deliveries, whethervaginal or cesarean, and cesarean deliveries in particular,do increase the likelihood of future placenta previa. Infact, women with the combination of high parity andmultiple repeat cesarean deliveries have the greatestlikelihood of placenta previa.

The relationship between multiple prior cesarean de-liveries and placenta previa is particularly important toconsider given the renewed controversy regarding thebenefits of a vaginal trial of labor after prior cesareandelivery.10 Repeat cesarean delivery has been associatedwith increased health care costs and maternal morbiditywhen compared with a vaginal trial of labor.11,12 As

shown in this study, the increase in maternal morbiditycaused by repeat cesarean delivery is not limited toimmediate operative complications but extends through-out a womans reproductive life.

REFERENCES

1. Iyasu S, Saftlas AK, Rowley DL, Koonin LM. The epide-miology of placenta previa in the United States, 1979through 1987. Am J Obstet Gynecol 1993;168:14249.

2. Zhang J, Savitz DA. Maternal age and placenta previa: Apopulation-based, case-control study. Am J ObstetGynecol 1993;168:6415.

3. Handler AS, Mason ED, Rosenberg DL, Davis FG. Therelationship between exposure during pregnancy to ciga-rette smoking and cocaine use and placenta previa. Am JObstet Gynecol 1994;170:8849.

4. Barrett JM, Boehm FH, Killam AP. Induced abortion: Arisk factor for placenta previa. Am J Obstet Gynecol 1981;141:76972.

5. Miller DA, Chollett JA, Goodwin TM. Clinical risk factorsfor placenta previa-placenta accreta. Am J Obstet Gynecol1997;177:2104.

6. McMahon MJ, Rongling L, Schenck AP, Olshan AF,Royce RA. Previous cesarean birth, a risk factor for pla-centa previa? J Reprod Med 1997;7:40912.

7. To WWK, Leung WC. Placenta previa and previouscesarean section. Intl J Gynecol Obstet 1995;51:2531.

8. Clark SL, Koonings PP, Phelan JP. Placenta previa/accretaand prior cesarean section. Obstet Gynecol 1985;66:8992.

9. Monica G, Lilja C. Placental previa, maternal smoking andrecurrence risk. Acta Obstet Gynecol Scand 1995;74:3415.

10. Greene M. Vaginal delivery after cesarean sectionIs therisk acceptable? N Engl J Med 2001;345:545.

11. Grobman WA, Peaceman AM, Socol ML. Cost-effective-ness of elective cesarean delivery after one prior lowtransverse cesarean. Obstet Gynecol 2000;95:74551.

12. Flamm BL, Goings JR, Liu Y, Wolde-Tsadik G. Electiverepeat cesarean delivery versus trial of labor: A prospec-tive multicenter study. Obstet Gynecol 1994;83:92732.

Address reprint requests to: Melissa Gilliam, MD, MPH, Uni-versity of Illinois at Chicago, Department of Obstetrics andGynecology, 820 South Wood Street MC 808, Chicago, IL60612; E-mail: Mgilli2@uic.edu.

Received September 18, 2001. Received in revised form January 11,2002. Accepted February 14, 2002.

Table 5. The Association Between the Number of PriorCesarean Deliveries and Placenta Previa Basedon Three Indicator Variables for Number of PriorCesarean Deliveries* (Adjusted Odds Ratios and95% Confidence Intervals Stratified by Parity)

Number ofprior cesarean

deliveries

Parity

1(n 1171)

2(n 675)

3(n 436)

0 1.0 1.0 1.01 1.28 1.10 1.69

0.82, 1.99 0.47, 2.55 0.74, 3.902 2.02 3.93

1.16, 3.53 1.37, 11.263 4.05

1.63, 10.10* Age 35 vs other, smoker vs nonsmoker, and any prior inducedabortion are included in the adjusted model.

The sample sizes for each parity level differ from those shown inTable 1 because of the presence of missing values.

980 Gilliam et al Cesarean Delivery and Placenta Previa OBSTETRICS & GYNECOLOGY