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 No attempts were made to locate any unpublished studies.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 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.
PIP: To quantify the risk of placenta previa based on the presence and number of cesarean deliveries and a history of spontaneous and induced abortion, a meta-analysis was conducted of salient studies published in the world literature in 1950-96. Subgroup analyses were conducted to identify important sources of heterogeneity by study design and location. The tabulation of 36 studies identified a total of 3.7 million pregnant women, of whom 13,992 had placenta previa. The reported incidence of placenta previa in these studies ranged from 0.28% to 2.0%. Women with at least 1 cesarean section delivery were 2.6 times (95% confidence interval (CI), 2.3-3.0) more likely to develop placenta previa in a subsequent pregnancy. The relative risk of placenta previa was higher in case-control studies (3.8) than cohort studies (2.4). An analysis of four studies, encompassing 170,640 pregnant women, showed a dose-response pattern for the risk of placenta previa on the basis of the number of previous cesarean deliveries. The relative risks increased from 4.5 (95% CI, 3.6-5.5) for 1 prior cesarean delivery to 44.9 (95% CI, 13.5-149.5) for 4 or more previous cesareans. Women with a history of spontaneous abortion had a relative risk of placenta previa of 1.6 (95% CI, 1.0-2.6), while those with a history of induced abortion had a relative risk of 1.7 (95% CI, 1.0-2.9). These findings suggest that pregnant women with a history of cesarean delivery or abortion should be regarded as at high risk for placenta previa and monitored closely. They further underscore the importance of avoiding unnecessary cesarean deliveries and encouraging vaginal birth for women with prior cesarean delivery.