The association of placenta previa with history of cesarean delivery and abortion: A metaanalysis

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November 1997 • Volume 177 • Number 5

The association of placenta previa with history of cesarean
delivery and abortion: A metaanalysis
Cande V. AnanthPhD, MPHa [MEDLINE LOOKUP]
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Abstract
TOP

Objective: Our purpose was to determine the incidence of placenta previa based on the
available epidemiologic evidence and to quantify the risk of placenta previa based on the
presence and number of cesarean deliveries and a history of spontaneous and induced
abortion.
Study Design: We reviewed studies on placenta previa published between 1950 and 1996 on
the basis of a comprehensive literature search with use of MEDLINE and by identifying studies
cited in the references of published reports. Studies were chosen for inclusion in the
metaanalysis if the incidence of placenta previa and its cross-classification with either prior
cesarean delivery or abortions (both spontaneous and induced) or both were available. We also
extracted details about the study design (case-control or cohort study) and place where they
were conducted (United States or other countries). Published case reports dealing with placenta
previa and studies relating to abruptio placentae were excluded from this review. We also
restricted the search to studies published in English. No attempts were made to locate any
unpublished studies. Data from studies identified during the literature search were reviewed and
abstracted by a single author. In case of discrepancies or when the information presented in a
study was unclear, abstraction by a (blinded) second reviewer was sought to resolve the
discrepancy.
Results: Data on the incidence of placenta previa and its associations with previous cesarean
delivery and abortions were abstracted. Subgroup analyses were performed to identify potential
sources of heterogeneity by study design and place where they were conducted. Statistical
methods used for the metaanalysis included the fixed-effects logistic regression model, whereas
potential sources of heterogeneity among studies were evaluated by fitting random-effects

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models. The tabulation of 36 studies identified a total of 3.7 million pregnant women, of whom
13,992 patients were diagnosed with placenta previa. The reported incidence of placenta previa
ranged between 0.28% and 2.0%, or approximately 1 in 200 deliveries. Women with at least
one prior cesarean delivery were 2.6 (95% confidence interval 2.3 to 3.0) times at greater risk
for development of placenta previa in a subsequent pregnancy. The results varied by study
design, with case-control studies showing a stronger relative risk (relative risk 3.8, 95%
confidence interval 2.3 to 6.4) than cohort studies did (relative risk 2.4, 95% confidence interval
2.1 to 2.8). Four studies, encompassing 170,640 pregnant women, provided data on the
number of previous cesarean deliveries. These studies showed a dose-response pattern for the
risk of previa on the basis of the number of prior cesarean deliveries. Relative risks were 4.5
(95% confidence interval 3.6 to 5.5) for one, 7.4 (95% confidence interval 7.1 to 7.7) for two, 6.5
(95% confidence interval 3.6 to 11.6) for three, and 44.9 (95% confidence interval 13.5 to 149.5)
for four or more prior cesarean deliveries. Women with a history of spontaneous or induced
abortion had a relative risk of placenta previa of 1.6 (95% confidence interval 1.0 to 2.6) and 1.7
(95% confidence interval 1.0 to 2.9), respectively. Substantial heterogeneity in the results of the
metaanalysis was noted among studies.
Conclusion: There is a strong association between having a previous cesarean delivery,
spontaneous or induced abortion, and the subsequent development of placenta previa. The risk
increases with number of prior cesarean deliveries. Pregnant women with a history of cesarean
delivery or abortion must be regarded as high risk for placenta previa and must be monitored
carefully. This study provides yet another reason for reducing the rate of primary cesarean
delivery and for advocating vaginal birth for women with prior cesarean delivery.

(Click on a term to search this journal for other articles containing that term.)
Key words: Cesarean delivery, induced abortion, metaanalysis, placenta previa,
spontaneous abortion


Although placenta previa is relatively uncommon (incidence of 3 to 9 per 1000 pregnancies), it
is regarded as one of the leading causes of uterine bleeding during the latter stages in
gestation1 and has been recognized as an important determinant of maternal morbidity and
adverse perinatal outcomes. Pregnancies complicated by placenta previa have resulted in
excessively high rates of preterm delivery, low birth weight, stillbirths, and neonatal and
perinatal deaths. Risk factors associated with placenta previa include advanced maternal age,
multiparity, cigarette smoking and “crack” or cocaine use, history of placenta previa, cesarean
delivery, spontaneous and induced abortions, and prior gynecologic surgeries. Nonetheless, the
etiology of placenta previa largely remains obscure and speculative. In spite of the advent of
ultrasonography to diagnose this disorder and the ability to assess fetal lung maturity to
appropriately time delivery, efforts to improve perinatal outcomes in cases of placenta previa
continue to pose a challenge.
It appears that the rate of cesarean delivery has been increasing steadily over the past two
decades. Some studies have observed an increased frequency of placenta previa among
women with a prior history of cesarean delivery or abortions, suggesting an association with
surgical procedures that disrupt the uterine cavity. Nonetheless, the extent to which a history of
cesarean delivery or spontaneous and induced abortion predisposes women to the
development of placenta previa is unclear from earlier studies.
We performed a systematic review and metaanalysis of all published studies on placenta previa
to determine its incidence and to quantify the risk of placenta previa on the basis of the
presence and number of cesarean deliveries and a history of spontaneous and induced
abortions. In addition, the systematic review of all studies enabled us to also identify sources of
heterogeneity among studies.

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Material and methods
TOP

Literature review.
We reviewed all studies published between 1950 and 1996 on placenta previa. Studies chosen
for the review were selected on the basis of a comprehensive literature search with use of
MEDLINE and by identifying studies cited in the bibliography of published reports. Key words
that were used in the MEDLINE search included “placenta pr(a)evia,” “placental disorders,”
“antepartum h(a)emorrhage,” and “antepartum bleeding.” In addition, the key words
“c(a)esarean delivery,” “c(a)esarean section,” “uterine surgery,” “spontaneous abortion,”
“induced abortion,” and “elective abortion” were also used in conjunction with the search leading
to studies on placenta previa. Published case reports dealing with placenta previa and studies
relating to abruptio placentae were excluded from this review. We also restricted the search to
studies published in English. No attempts were made to locate any unpublished studies or
studies in abstract form. Multiple articles resulting from the same data source (e.g.,
Collaborative Perinatal Project2-5) were only included once in the metaanalysis. However, if two
studies came from the same data source but spanned nonoverlapping time periods of data
accrual, they were both included in the metaanalysis.
Data extraction.
We identified a total of 41 published studies2,6-45 on the basis of our inclusion criteria relating to
placenta previa. From these studies, information on the incidence of placenta previa or its
association with history of cesarean delivery and abortions were available from 36 studies.2,6-
40These studies were reviewed critically by the first author (C.V.A.), and information on the total
number of pregnancies and the number of pregnancies complicated by placenta previa and
data on placenta previa cross-classified by prior cesarean delivery and spontaneous and
induced abortions were abstracted. Data on the type of study design (i.e., cohort or case-control
studies) and the country where the study was carried out were also ascertained. Studies20,41-43
that did not provide sufficient information to carry out a metaanalysis or those that did not
provide data for the comparison group (e.g., number of pregnancies without placenta previa
among women with prior cesarean delivery) were excluded from our metaanalysis, although
these studies were still included in an analysis of incidence of previa, when data were available.
In case of discrepancies or when the information presented in a study was unclear, abstraction
by a (blinded) second reviewer (J.C.S.) was sought to resolve the discrepancy.
Statistical methods for metaanalysis.
The incidence (risk) of placenta previa from cohort studies was obtained by dividing the number
of cases of placenta previa by the total number of pregnancies. This information was abstracted
from case-control studies, if reported. We calculated odds ratios and their SEs as the effect
measure on the basis of the data abstracted from each study. Data that were abstracted from
each study were arranged in 2 × 2 tables, and 0.5 was added to cells that contained no
observations to improve the precision of the effect measure.46 Pooled estimates of odds ratios
were obtained by weighting each study by the inverse variance of the effect measure on a
logarithmic scale. This approach to pooling the results assumes that the study populations
being compared are similar and hence corresponds to a fixed-effects analysis. The validity of
pooling the odds ratios was tested (test for heterogeneity) on the basis of a 2 test.47 A violation
of this test implies that the studies being grouped differ from one another. In the presence of
significant heterogeneity in the effect measure among studies being compared, we then
performed a random-effects analysis that was based on the method described by DerSimonian
and Laird.48 The random-effects analysis accounts for the interstudy variations. Our goal of
performing this metaanalysis was to identify sources of heterogeneity among studies.
To examine for the presence of any trends in the incidence of placenta previa over time, we
used the locally weighted scatterplot smoother procedure.49 This procedure is a nonparametric
scatterplot smoother that down-weights observations that are distant from its neighbors and,

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conversely, assigns larger weights to observations that are closer to each other. For
metaanalysis relating to the number of prior cesarean deliveries, we computed an estimate of
log odds ratio with their SEs on the basis of methods described by Greenland and
Longnecker.50 This method adjusts for the correlation that results from use of a single reference
category (i.e., no prior cesarean delivery) while evaluating the risk of placenta previa by number
of prior cesarean deliveries.
To assess the public health implications of history of cesarean delivery and abortions on
placenta previa, we also computed the population attributable risk.51 The population attributable
risk can be interpreted as the proportion of the adverse outcome (i.e., placenta previa) that
could be attributed to cesarean delivery or spontaneous and induced abortions. All statistical
analyses were performed on the SAS system version 6.11 (SAS Institute, Cary, N.C.) operating
on the UNIX system.

Results
TOP

Incidence of placenta previa.
Data abstracted from 36 studies2,6-40 resulted in a total of 3.7 million pregnant women, of whom
13,992 were identified with placenta previa. The reported incidence of placenta previa ranged
between 0.28% to 1.96%, or approximately 1 in 200 pregnancies. The incidence of placenta
previa was the same for both cohort and case-control studies. An examination for trends over
time in the incidence of placenta previa revealed that the incidence of this disorder was almost
similar until the mid-1980s (1966 to 1974: incidence was 0.36%; 1975 to 1984, 0.37%), but the
incidence was 0.48% among studies conducted between 1985 and 1995 (Fig. 1).
Fig. 1. Trends in incidence of placenta previa (smoothed line was
generated based ! on locally weighted scatterplot smoother
procedure, a nonparametric regression smoothing procedure).

Click on Image to view full size
Association with prior cesarean delivery.
Associations between history of cesarean delivery and placenta previa were evaluated in 15
published studies6-18,44,45 (Fig. 2).
Fig. 2. Association of placenta previa with history of cesarean
delivery: Odds ratios with 95% confidence intervals (reference
numbers in brackets).

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Click on Image to view full size
Pooling of data from all these studies resulted in an odds ratio of 2.6 (95% confidence interval
2.3 to 3.0), although the test for homogeneity of the pooled odds ratios was violated ( 2 = 222.8,
14 degrees of freedom, p < 0.0001) (Table I).

Table I. Association between prior cesarean delivery and placenta previa: A
metaanalysis based on fixed- and random-effects models

No. of
Fixed-effects OR and
Random-effects OR and
Comparison
studies
95% CI
2*significance
95% CI
Overall6-18,41,42
15
2.9 (2.8-3.0)
222.8 (p < 0.0001)
2.6 (2.3-3.0)
U.S. studies7,10,11,15,16,18,42
7
2.2 (1.9-2.5)
17.8 (p < 0.0001)
2.3 (1.7-2.8)
Other countries6,8,9,12-15,17,41
8
2.9 (2.8-3.0)
184.6 (p < 0.0001)
2.4 (2.0-2.9)
Cohort studies6,7,9,11,12,16,17
8
2.9 (2.8-3.0)
174.4 (p < 0.0001)
2.4 (2.1-2.8)
Case-control studies8,10,13,15,18,41,42
7
2.6 (2.2-3.1)
46.9 (p < 0.0001)
3.8 (2.3-6.4)

OR, Odds ratio; CI, confidence interval.
*Degrees of freedom for 2 test are number of studies minus one.

Hence stratification of the studies on the basis of their study designs resulted in a pooled odds
ratio of 2.4 (95% confidence interval 2.1 to 2.8) for cohort6,7,10-12,16,17 and 3.8 (95% confidence
interval 2.3 to 6.4) for case-control studies.8,10,13,15,18,41,42 Tests for homogeneity of odds ratios
for both comparisons were violated. Associations between prior cesarean delivery and placenta
previa were almost the same both for studies based in the United States6-9,11-14,17,41 and
elsewhere.7, 10, 11, 15, 16, 42
We further analyzed the association between placenta previa in relation to the number of prior
cesarean deliveries. Data available from four studies7,12,14,17 encompassing a total of 170,640
pregnant women showed a dose-response pattern in the risk of previa with increasing number
of prior cesarean deliveries. Odds ratios were 4.5 (95% confidence interval 3.6 to 5.5) for one
and 7.4 (95% confidence interval 7.1 to 7.7) for two prior cesarean deliveries (based on all four
studies), 6.5 (95% confidence interval 3.6 to 11.6) for three prior cesarean deliveries (based on
two studies7,17), and 44.9 (95% confidence interval 13.5 to 149.5) for four or more prior
cesarean deliveries (based on one study7). There was an exponential increase in the risk of
placenta previa with number of prior cesarean deliveries.
Association with prior abortions.
We identified five studies15,18,28,52,53 that evaluated the association between prior spontaneous
abortion and the subsequent development of placenta previa (Fig. 3).
Fig. 3. Association of placenta previa with history of spontaneous
abortion: Odds ratios with 95% confidence intervals (reference
numbers in brackets).

Click on Image to view full size
The random-effects pooled analysis indicated that the risk of placenta previa was 1.7 (95%
confidence interval 1.0 to 2.9) for women with at least one prior spontaneous abortion (Table
II).

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Table II. Association between prior spontaneous and induced abortions and
placenta previa: A metaanalysis based on fixed- and random-effects models

Fixed-effects OR and
Random-effects OR and
Comparison
No. of studies
95% CI
2p value
95% CI
Spontaneous abortion†




Overall15,18,28,52,53
5
1.7 (1.5-2.0)
3.4 (0.4933)
1.7 (1.0-2.9)
Cohort studies28
1
3.0 (1.5-2.0)
—
—
Case-control studies15,18,52,53
4
1.7 (1.5-2.0)
2.5 (0.4753)
1.7 (1.5-2.0)
Induced abortion†




Overall9,15,28,42,52,54
6
1.5 (1.3-1.7)
43.7 (p < 0.0001)
1.6 (1.0-2.6)
Cohort studies28
1
6.7 (2.7-16.8)
—
—
Case-control studies9,15,42,52,54
5
1.5 (1.3-1.7)
33.3 (p < 0.0001)
1.3 (0.7-2.3)

OR, Odds ratio; CI, confidence interval.
*Degrees of freedom for 2 test is number of studies minus one.
†All studies were based in the United States.

All five studies were cohort studies. The test for homogeneity of odds ratios was not violated (p
= 0.4933). The association between prior spontaneous abortion and the subsequent
development of placenta previa was the same when stratified on study design.
Six studies10,15,28,42,52,54 reported the association between placenta previa and history of induced
abortions (Fig. 4), all of whom were based in the United States.
Fig. 4. Association of placenta previa with history of induced
abortion: Odds ratios with 95% confidence intervals (reference
numbers in brackets).

Click on Image to view full size
Five of the six studies were case-control studies. Although the test for homogeneity of the
pooled odds ratio was violated ( 2 = 43.7, 5 degrees of freedom, p < 0.0001), a history of
spontaneous abortion was associated with a 70% (95% confidence interval 1.0 to 2.9) increase
in the risk of subsequent development of placenta previa. Restricting the analysis to the five
case-control studies10,15,42,52,54 resulted in a decreased odds ratio (odds ratio 1.3, 95%
confidence interval 0.7 to 2.3) for placenta previa, whereas the odds ratio from the single cohort
study28 was 6.7 (95% confidence interval 2.7 to 16.8). Additionally, with use of data from the
vital records, Zhang and Savitz reported that the risk of placenta previa was 1.6 (95%
confidence interval 1.3 to 1.8) for women with one abortion (either spontaneous or induced), 2.3
(95% confidence interval 1.8 to 3.0) for those with two, and 3.7 (95% confidence interval 2.7 to
5.2) for those with three or more abortions.

Comment
TOP
Placenta previa has been reported to occur in approximately 0.3% to 0.8% of pregnancies.1 A
variation in this incidence has been attributed to methods of diagnosis, definitions used, and
diverse nature of patient populations being studied. Although the overall incidence of placenta
previa has been remarkably stable for almost three decades, the incidence of this disorder was
almost similar until the mid-1980s (1966 to 1974: incidence was 0.36%; 1975 to 1984, 0.37%),
but the incidence was 0.48% among studies conducted between 1985 and 1995 (see Fig. 1).

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The increased incidence of placenta previa in the last decade may be the result of increasing
cesarean delivery rates during this period or the more widespread use of ultrasonography for
detecting placenta previa.
Several studies, based on ultrasonography findings, have shown that the incidence of placenta
previa is about 3% to 5% in a normal obstetric population during midtrimester.55 However, this
frequency falls dramatically to almost 0.3% to 0.7% among term pregnancies as a result of the
so-called placental “migration.” Almost four decades ago Bender56 first observed an increased
frequency of placenta previa among women with uterine scarring (because of cesarean delivery
or abortions) in prior pregnancies. Recently, few studies have explored this association and
have unequivocally observed increased risks of placenta previa among women with a history of
cesarean delivery. An association between placenta previa and prior cesarean delivery is
biologically plausible. Damage to the endometrial and myometrial uterine lining (during
cesarean delivery) can predispose to a low implantation of the placenta in the uterus. This
metaanalysis quantifies the risk on the basis of the number of previous cesarean deliveries,
implying a dose-response effect of multiple uterine procedures. Likewise, curettage of the
uterus during a spontaneous or induced abortion may significantly damage the endometrium
and uterine cavity so as to increase the risk for placenta previa. Unfortunately, we were unable
to evaluate the association between curettage and subsequent development of placenta previa
because of insufficient information from published studies.
The strength of this metaanalysis is the sheer size of the study. Although some of the
associations observed in this study violated the homogeneity assumptions because of pooling
of several studies, interstudy heterogeneity was adequately addressed through the fit of models
on the basis of random-effects analysis. Nonetheless, a few methodologic limitations in this
study must also be noted. First, the risk of placenta previa increases both because of aging
effects of the uterus and repeated pregnancies.39 Hence an association between prior uterine
scarring from cesarean delivery or abortion and placenta previa may have been confounded, in
part, because of repeated pregnancies. Unfortunately, insufficient data from published studies
precluded us from adequately controlling for parity effect. Another potential limitation of the
metaanalysis is publication bias. We may have missed identifying published studies during our
literature search. In spite of our best efforts to identify studies, we may have missed some that
may have reported data on placenta previa and prior cesarean delivery or abortions as a
secondary analysis. Nevertheless, to account for this bias on our findings, we report the results
from random-effects regression models, which assume that studies included in our
metaanalysis is a (random) sample from a larger population of similar studies. In addition, we
generated a “funnel-plot” (by plotting the log odds ratio against their corresponding SEs, graph
not shown) to examine for indications of publication bias for history of cesarean delivery and
abortions. The plots did not indicate the presence of any publication bias.
Another shortcoming of this metaanalysis is that we were unable to evaluate the risk of placenta
previa in relation to uterine rupture, complications occurring with an abortion, curettage for
postpartum hemorrhage or retained products of conception, postpartum complications such as
endometritis, multiple gestations, and other potential risk factors for this disorder. These data
were not universally available from each of the individual studies. Future prospective studies
should evaluate these factors to better understand the etiology of placenta previa.
Public health implications.
The rates of primary cesarean delivery have been steadily increasing in the past decade.
Although this increase has probably improved fetal and neonatal morbidities and other adverse
reproductive outcomes as well, the public health implications for the rise in cesarean delivery
rates have been poorly addressed.
Given that the rate of cesarean delivery is 20% in the general population and if one is interested
in reducing this rate by 50% (i.e., from 20% to 10%), the population-attributable risk51 for prior
cesarean delivery on the subsequent risk of placenta previa is 14%. This implies that by
reducing the primary and repeat cesarean delivery rates by half the risk for placenta previa
could be reduced by 14%. Similarly, assuming that the rates of spontaneous and induced

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abortions are 20% and 5%, respectively, and if these rates are reduced by 50%, then 6.5% and
1.5% of placenta previa cases could potentially be averted. This suggests that a reduction in
uterine instrumentation rates for the management of both spontaneous and induced abortions
could further reduce the risk of placenta previa. The public health ramifications of these findings
needs careful assessment in future prospective studies.
In conclusion, our (meta)analysis clearly demonstrates an elevated risk for placenta previa
among women with prior cesarean delivery or abortions. Moreover, this risk increases
dramatically with increasing number of prior cesarean deliveries. Pregnant women with a history
of cesarean delivery or abortion must be regarded as being at increased risk for the subsequent
development of placenta previa. This study provides yet another reason for reducing the
primary cesarean delivery rate and for advocating vaginal birth for women with prior cesarean
delivery.

References
TOP
1. Cunningham GF, MacDonald PC, Gant NF, Leveno KJ, Gilstrap LC, editors.
Williams' obstetrics. 19th ed. Norwalk (CT): Appleton & Lange; 1993. p. 819-51.

2. Niswander KR, Friedman EA, Hoover DB, Pietrowski H, Westphal M. Fetal
morbidity following potentially anoxigenic obstetric conditions. II. Placenta previa. Am J
Obstet Gynecol 1966;95:846-52.
MEDLINE


3. Naeye RL. Effects of maternal smoking on the fetus and placenta. Br J Obstet
Gynaecol 1978;85:732-7.
MEDLINE


4. Naeye RL. Abruptio placentae and placenta previa: frequency, perinatal mortality,
and cigarette smoking. Obstet Gynecol 1980;55:701-4.
MEDLINE


5. Naeye RL. Placenta previa: predisposing factors and effects on the fetus and
surviving infants. Obstet Gynecol 1978;52:521-5.
MEDLINE


6. Singh DM, Rodriguez C, Gupta AM. Placenta previa and previous cesarean section.
Acta Obstet Gynecol Scand 1981;60:367-8.
MEDLINE


7. Clark SL, Koonings PP, Phelan JP. Placenta previa/accreta and prior caesarean
section. Obstet Gynecol 1985;66:89-92.
MEDLINE


8. Rose GL, Chapman MG. Aetiological factors in placenta previa—a case-controlled
study. Br J Obstet Gynaecol 1986;93:586-8.
MEDLINE


9. Nielsen TF, Hagberg H, Ljungbald U. Placenta previa and antepartum hemorrhage
after previous caesarean section. Gynecol Obstet Invest 1989;27:88-90.
MEDLINE


10. Williams MA, Mittendorf R, Leiberman E, Monson RR, Schoenbaum SC, Genest
DR. Cigarette smoking during pregnancy in relation to placenta previa. Am J Obstet
Gynecol 1991;165:28-32.

---

MEDLINE


11. Zhang J, Savitz DA. Maternal age and placenta previa: a population-based case-
control study. Am J Obstet Gynecol 1993;166:641-5.

12. Parazzini F, Dindelli M, Luchini L, La Rosa M, Potenza MT, Frigerio L, et al. Risk
factors for placenta previa. Placenta 1994;15:321-6.
MEDLINE


13. Maksheed M, El-Tomi N, Moussa M. A retrospective analysis of pathological
placental implantation—site and penetration. Int J Obstet Gynecol 1994;47:127-34.

14. Chattopadhyay SK, Kharif H, Sherbeeni MM. Placenta previa and accreta after
previous cesarean section. Eur J Obstet Gynecol Reprod Biol 1993;52:151-6.
MEDLINE


15. Handler AS, Mason ED, Roseberg DL, Davis FG. The relationship between
exposure during pregnancy to cigarette smoking and cocaine use and placenta previa.
Am J Obstet Gynecol 1994;170:884-9.
MEDLINE

ABSTRACT

FULL TEXT


16. Thomas AG, Alvari M, Friedman S Jr, Brodman ML, Kim J, Lockwood C. The
effect of placenta previa on blood loss in second trimester pregnancy termination.
Obstet Gynecol 1994;84:58-60.
MEDLINE


17. Hershkowitz R, Fraser D, Mazor M, Leiberman JR. One or multiple previous
cesarean sections are associated with similar increased frequency of placenta previa.
Eur J Obstet Gynecol Reprod Biol 1995;62:185-8.
MEDLINE


18. Chelmow D, Andrew E, Baker ER. Maternal cigarette smoking and placenta
previa. Obstet Gynecol 1996;87:703-6.
MEDLINE


19. McShane PM, Heyl PS, Epstein MP. Maternal and perinatal morbidity resulting
from placenta previa. Obstet Gynecol 1985;65:176-82.
MEDLINE


20. Silver R, Depp R, Sabbagha RE, Dooley SL, Socol ML, Tamura RK. Placenta
previa: aggressive expectant management. Am J Obstet Gynecol 1984;150:15-22.
MEDLINE


21. Lilja GMC. Placenta previa, maternal smoking and recurrence risk. Acta Obstet
Gynecol Scand 1995;74:341-5.
MEDLINE


22. Hemminki E, Glebatis DM, Therriault GD, Janerich DT. Incidence of placenta
previa and abruptio placentae in New York State. N Y State J Med 1987;87:594-8.
MEDLINE


23. Chervenak F, Lee Y, Handler MA, Monoson RF, Berkowitz RL. Role of attempted
vaginal delivery in the management of placenta previa. Obstet Gynecol 1984;64:798-
801.

---

MEDLINE


24. MacGillivray I, Davey D, Isaac S. Placenta previa and sex ratio at birth. BMJ
1986;292:371-2.

25. Iyasu S, Saftlas AK, Rowley DL, Koonin LM, Lawson HW, Atrash HK. The
epidemiology of placenta previa in the United States, 1979 through 1987. Am J Obstet
Gynecol 1993;168:142-9.

26. Cotton DB, Reid JA, Paul RH, Culligan EJ. The conservative aggressive
management of placenta previa. Am J Obstet Gynecol 1980;137:687-95.
MEDLINE


27. Brenner WE, Edelman DA, Hendricks CH. Characteristics of patients with placenta
previa and results of “expectant management.” Am J Obstet Gynecol 1978;132:180-91.
MEDLINE


28. Barrett JM, Boehm FH, Killam AP. Induced abortion: a risk factor for placenta
previa. Am J Obstet Gynecol 1981;141:769-72.
MEDLINE


29. Neri A, Gorodeski I, Bahari C, Ovadia Y. Impact of placenta previa on intrauterine
fetal growth. Isr J Med Sci 1980;16:429-32.
MEDLINE


30. Crenshaw C Jr, Jones DED, Parker RT. Placenta previa: a survey of twenty years
experience with improved perinatal survival by expectant therapy and cesarean
delivery. Obstet Gynecol Surv 173;28:416-7.

31. Wexler P, Gottesfeld KR. Early diagnosis of placental previa. Obstet Gynecol
1979;54:231-4.
MEDLINE


32. Parsons MT, Wineger A, Siefert L, Spellacy WN. Pregnancy outcomes in short
women. J Reprod Med 1989;34:357-61.
MEDLINE


33. Jakobovits M, Zubek L. Sex ratio and placenta previa. Acta Obstet Gynecol Scand
1989;68:503-5.
MEDLINE


34. Khashoggi T. Maternal and neonatal outcome in major placenta previa. Ann Saudi
Med 1995;15:313-6.

35. Wolf EJ, Mallozzi A, Rodis JF, Egan JFX, Vintzileos AM, Campbell WA. Placenta
previa is not an independent risk factor for small for gestational age infant. Obstet
Gynecol 1991;77:707-9.
MEDLINE


36. Leiberman JR, Frasier D, Kasis A, Mazor M. Reduced frequency of hypertensive
disorders in placenta previa. Obstet Gynecol 1991;77:83-6.
MEDLINE


37. Ananth CV, Wilcox AJ, Savitz DA, Bowes WA Jr, Luther ER. Influence of maternal

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