1666
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wileyonlinelibrary.com/journal/aogs Acta Obstet Gynecol Scand. 2020;99:1666–1673.Received: 19 January 2020
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Revised: 9 June 2020|
Accepted: 16 June 2020 DOI: 10.1111/aogs.13943O R I G I N A L R E S E A R C H A R T I C L E
Placental location and risk of retained placenta in women with a previous cesarean section: A population-based cohort study
Michaela Granfors
1,2| Anna Sandström
1,2,3,4| Olof Stephansson
1,2| Johanna Belachew
3| Ove Axelsson
3,5| Anna-Karin Wikström
3© 2020 The Authors. Acta Obstetricia et Gynecologica Scandinavica published by John Wiley & Sons Ltd on behalf of Nordic Federation of Societies of Obstetrics and Gynecology (NFOG)
Abbreviations: aOR, adjusted odds ratio; BMI, body mass index; CI, confidence interval; OR, odds ratio.
1Department of Medicine, Solna, Clinical Epidemiology Division, Karolinska Institutet, Stockholm, Sweden
2Department of Women’s Health, Division of Obstetrics, Karolinska University Hospital, Stockholm, Sweden
3Department of Women's and Children's Health, Uppsala University, Uppsala, Sweden
4Department of Obstetrics and Gynecology, Oregon Health and Science University, Portland, OR, USA
5Center for Clinical Research, Sörmland, Uppsala University, Eskilstuna, Sweden Correspondence
Michaela Granfors, Department of Medicine, Solna, Clinical Epidemiology Division, Karolinska Institutet, 171 77 Stockholm, Sweden.
Email: michaela.granfors@ki.se Funding information
Olof Stephansson was supported by the Swedish Research Council and Karolinska Institutet.
Abstract
Introduction: Some studies have shown that women with a previous cesarean sec- tion, compared with women with a previous vaginal delivery, have an increased risk of retained placenta during a subsequent vaginal delivery. It is unknown whether this is mediated by anterior placental location, when the placenta might cover the uterine scar. The aim of this study was to evaluate whether the increased risk of retained placenta in women with a previous cesarean section is mediated by anterior placental location.
Material and methods: This is a population-based cohort study, with data from the regional population-based Stockholm-Gotland Obstetric Cohort, Sweden, from 2008 to 2014. The overall study population included 49 598 women with a vaginal sec- ond delivery, where adequate information about placental location from the second- trimester ultrasound scan was available. For the main analysis, including the 3921 women with a previous cesarean section, we calculated the relative risk of retained placenta in women with an anterior placental location, using women with non-ante- rior placental locations as reference. Relative risks were calculated as odds ratios (OR) with 95% CI. In a second model, adjustments were made for maternal age, height, country of birth, smoking in early pregnancy, infant sex, and in vitro fertilization.
Results: In the overall study population, the rate of retained placenta at the second delivery was 2.0%. The proportion of women with a retained placenta was higher among women with a previous cesarean compared with those with a previous vagi- nal delivery (3.4% vs 1.9%; P < .0001). In the main analysis, including women with a previous cesarean section, the risk for retained placenta was not increased with anterior compared with non-anterior placental location (OR 0.84, 95% CI 0.60-1.20).
Adjustments did not affect the estimates in a significant way.
Conclusions: The increased risk of retained placenta in women with a previous cesar- ean section is not mediated by anterior placental location.
This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.
1 | INTRODUCTION
The rates of cesarean section are increasing worldwide.1 As a con- sequence, complications in subsequent pregnancies and deliveries, such as placenta previa and abnormal invasive placenta, are increas- ing concurrently.2 A vaginal birth after one cesarean is considered to be associated with fewer complications compared with elective repeat cesarean.3 On the other hand, a trial of labor, which ends with a repeat cesarean section, entails higher risks for complications.3 However, there might be complications in vaginal birth after one ce- sarean that have not been evaluated.
Retained placenta after vaginal delivery is a severe complica- tion, primarily because of its association with postpartum hemor- rhage.4 A recent population-based study showed that women with a previous cesarean section, compared with women with a previous vaginal delivery, had an increased risk of retained placenta during a subsequent vaginal delivery.5 The increased risk among women with a previous cesarean was most pronounced for retained pla- centa with severe postpartum hemorrhage. The reason for this as- sociation is not fully understood. Abnormal invasive placenta after previous cesarean section almost always occurs with placenta pre- via, and a strong tendency towards severe postpartum hemorrhage has been observed for anteriorly located placentas judged to cover the scar from the previous surgery in the uterus.6 It is unknown whether retained placenta in women with a previous cesarean sec- tion might be an early stage of abnormal invasive placenta and so occur more frequently in anterior placental location, when the pla- centa might cover the scar.
We conducted a population-based cohort study with the aim to evaluate whether the increased risk for retained placenta in women with a previous cesarean section is mediated by anterior placental location.
2 | MATERIAL AND METHODS
Data on maternal, pregnancy, delivery, infant, and antenatal ultra- sound characteristics were obtained from the population-based Stockholm-Gotland Obstetric Database.12 The database contains electronically transferred information from the electronic medical record system used in all antenatal, ultrasound, delivery, and post- natal care units in the Stockholm-Gotland region in Sweden, cover- ing about one fourth of all deliveries in Sweden. There were seven delivery hospitals in the region during the study period, with a total of approximately 25 000 annual births.
Antenatal care in Sweden is standardized, free of charge, and de- centralized to maternity care districts. Deliveries occur at hospitals and less than 0.01% are planned home deliveries. All pregnant women
are invited to a second-trimester ultrasound scan at 18-19 weeks of gestation, and approximately 97% of the pregnant population par- ticipates.7 One of the aims of the examination is to determine the location of the placenta.
The database includes information on 175 522 pregnancies with singleton, live-born infants from January 2008 through October 2014 (Figure 1). We included all women with a second delivery (n = 64 923), and excluded those with a second delivery by cesarean (n = 12 029). The overall study population consisted of all women with a vaginal second delivery, where adequate information about placental location from the second trimester ultrasound scan be- tween 16+0 and 22+6 weeks of gestation was available, and where those with placenta previa at the second-trimester ultrasound scan were excluded (n = 52). For the main analysis, we further excluded women with a previous vaginal delivery (n = 45 677). Hence, the main study population consisted of 3921 women with a vaginal sec- ond delivery and a previous cesarean section, and where adequate information about placental location from the second-trimester ul- trasound scan was available.
For description of placental location, there are three scrolling lists in the computerized electronic ultrasound medical record.
Each of the scrolling lists is optional to fill in, and there is no inter- nal hierarchy between the lists. In list (a), anterior and/or posterior location can be marked; in list (b), fundal, high, low, marginal, or previa location can be marked, and in list (c) left or right location can be marked. Information about placental location can also be reported in free text, but is then not available for analysis in the database. We categorized placental location in a descending hier- archy as follows: (a) fundal placenta (= “fundal” with or without ad- ditional information from other scrolling lists); (b) lateral placenta (= “left” or “right” placenta with or without additional information from the remaining scrolling lists); (c) anterior placenta (= “ante- rior” in the absence of posterior location, with or without addi- tional information from the remaining scrolling list) or (d) posterior placenta (= “posterior” in the absence of anterior location, with or without additional information from the remaining scrolling list).
For example, if “fundal” and “anterior” were indicated, the placen- tal location was classified as “fundal.” If “posterior” and “left” were indicated, the placental location was classified as “lateral.” If only K E Y W O R D S
placenta, placental location, previous cesarean, retained placenta, second trimester, ultrasound
Key Message
The increased risk of retained placenta in women with a previous cesarean section is not mediated by anterior pla- cental location.
“marginal” was indicated, the information about placental location was defined as insufficient. In women who underwent more than one second-trimester ultrasound examination during the time pe- riod, the latest scan with information about placental location was included.
For the analysis of rates of retained placenta at second delivery in relation to mode of first delivery in the overall study population, the exposure was previous cesarean section.
For the main analysis in women with a previous cesarean sec- tion, the exposure was placental location at the second-trimester ultrasound scan. We categorized placental location as “anterior” or
“non-anterior” (fundal, lateral or posterior). Non-anterior placental location was chosen as reference.
In a further subanalysis, non-anterior placental location was di- vided into posterior, fundal, and lateral location. Posterior placental location has in some studies been reported to be associated with a lower risk of complications than other placental locations, and was therefore chosen as reference category.5,6
The main outcome was retained placenta. Retained placenta was defined as a manual removal of the placenta according to the Nordic Medico-Statistical Committee (NOMESCO) Classification of surgical procedures, at discharge from the hospital (MBA30).8 The secondary outcome was retained placenta in combination with severe postpartum hemorrhage. Severe postpartum hemorrhage was defined as estimated blood loss >1000 mL during delivery and within the first 2 hours post- partum. Data were obtained from the standardized delivery records.
Covariates were maternal age, height, body mass index (BMI), smoking in early pregnancy, family situation, and country of birth, as well as pregnant by in vitro fertilization and infant sex. At the first antenatal visit in Sweden (usually at 7-12 gestational weeks), information about maternal characteristics and obstetric history is collected by self-reporting and registered in a standardized way.
Maternal weight is measured in light indoor clothing. Maternal age was collected at time of birth of the infant. Infant sex was collected from the standardized delivery records. The variables were catego- rized according to Table 1.
F I G U R E 1 Flow diagram of the study protocol
175 522 women, at least one pregnancy
within the register
64 923 women with a second delivery
Exclusion
• Parity ≠ 2, n=110 599
52 894 women with a vaginal second delivery
Exclusion
• Second delivery by cesarean, n= 12 029
s49 598 women with a vaginal second
delivery and adequate information about
placental location
(Overall study population)3921 women with a vaginal second delivery, adequate information about placental location and a previous cesarean delivery
(Main study population)
Exclusion
• No information in scrolling lists about placental location at ultrasound, n=3021
• placenta previa at ultrasound, n=52
• Insufficient information about placental location from scrolling lists, n=223
Exclusion
• Previous vaginal delivery, n= 45 677)
2.1 | Statistical analyses
In the overall study population, we estimated rates of retained pla- centa at second delivery in relation to mode of first delivery (vaginal vs cesarean section) by chi-squared test. For the main analysis in women with a previous cesarean section, we calculated odds ratios (OR) with 95% CI for retained placenta in women with anterior placenta, using non-anterior (fundal, lateral, or posterior) placental location as a refer- ence. Covariates were considered to be confounders when potentially having an impact on both placental location (exposure) and retained placenta (outcome). Based on our material, no confounders were pre- sent and crude OR might present the true association between pla- cental location and retained placenta. However, our cohort might be too small to detect significant associations between covariates and placental location and retained placenta, respectively. We therefore calculated a second, adjusted model (adjusted odds ratios; aORs) for retained placenta, controlling for maternal age, height, smoking in early pregnancy, and country of birth, as well as in vitro fertilization pregnancy and sex of the infant. These confounders were chosen based on a previous study on placental location and pregnancy out- comes in nulliparous women using the same database.9
Thereafter, in a subanalysis in women with a previous cesarean section, we calculated rates and OR for retained placenta in women with anterior, fundal, or lateral placental location using posterior lo- cation as the reference group.
Finally, we calculated rates and OR for retained placenta in combination with severe postpartum hemorrhage, according to pla- cental location. We performed analyses based on binary placental location (anterior vs non-anterior) and detailed placental location (anterior, posterior, fundal, and lateral), using the same reference groups and adjustments as described above. For adjusted analy- sis, only pregnancies with complete data on all covariates were in- cluded (n = 3698).
Statistical analyses were performed using the SAS software ver- sion 9.4 (www.sas.com). For adjusted analysis, only observations with complete information about all covariates were included (com- plete case analysis). A P-value of <.05 was considered statistically significant.
2.2 | Ethical approval
This study was approved by the regional ethical committee at Karolinska Institutet, Stockholm, Sweden (No. 2009/275-31 [2 April 2009], and No. 2012/365-32 [24 February 2012]).
3 | RESULTS
In the overall study population of women with a vaginal second de- livery, 92.1% (n = 45 677) had a previous vaginal delivery and 7.9%
(n = 3921) had a previous cesarean delivery. The overall rate of retained placenta at the second delivery was 2.0% (n = 979). The proportion
of women diagnosed with retained placenta was lower among women with a previous vaginal delivery (n = 845; 1.9%) than among women with a previous cesarean section (n = 134; 3.4% [P < .0001]). An an- terior placental location was slightly more common in women with a previous vaginal delivery compared with women with a previous ce- sarean section (46.9% and 45.1%, respectively [P = .03]).
TA B L E 1 Rates and odds ratios of retained placenta according to maternal characteristics and infant sex in women with a previous cesarean delivery (n = 3921)
Maternal characteristics
Number of births
Retained placenta Rates
Crude OR (95%
CI) Maternal age, y
<25 187 2.1 0.83 (0.28-2.46)
25-29 738 2.6 Reference
30-34 1666 3.5 1.39 (0.82-2.35)
≥35 1329 4.9 1.54 (0.90-2.63)
Missing 1
Maternal height, cm
<164 1345 2.8 0.81 (0.54-1.23)
164-171 1733 3.5 Reference
>171 815 4.2 1.21 (0.79- 1.87)
Missing 28
Maternal BMI, kg/m2
<18.5 67 3.0 0.94 (0.23-3.90)
18.5-24.9 2362 3.2 Reference
25-29.9 963 4.4 1.39 (0.95-2.04)
≥30 350 2.6 0.81 (0.40-1.62)
Missing 179
Smoking, early pregnancy
Yes 122 6.6 2.05 (0.98-4.29)
No 3775 3.3 Reference
Missing 24
Living with partner
Yes 3742 3.4 Reference
No 131 1.5 0.44 (0.11-1.79)
Missing 48
Maternal country of birth
Non-Nordic 876 2.9 0.82 (0.53-1.28)
Nordic 2871 3.5 Reference
Missing 174
In vitro fertilization
Yes 124 4.8 1.46 (0.63-3.37)
No 3797 3.4 Reference
Infant sex
Girl 1988 4.0 1.41 (1.00-2.01)
Boy 1933 2.9 Reference
Abbreviations: OR, odds ratio; CI, confidence interval; BMI, body mass index.
For the main analysis, the 3921 women with a previous cesarean section were included. Table 1 presents rates and odds ratios of re- tained placenta according to maternal characteristics and infant sex in women with a previous cesarean section. No significant differ- ences were observed.
Table 2 presents numbers and rates of placental location (anterior or non-anterior) according to maternal characteris- tics and infant sex in women with a previous cesarean section.
Non-anterior placenta location seemed to be more common in pregnancies with women with high BMI, but no other differences were observed. Hence, there were no statistical associations be- tween the covariates and retained placenta or placental location (except BMI).
Table 3 presents numbers, rates, and odds ratios of retained placenta according to placental location in women with a previous cesarean section. Compared with non-anterior placental location, Maternal characteristics Number
Placental location
P-value* Anterior
rates Non-anterior rates Maternal age, y
<25 187 42.8 57.2 .92
25-29 738 44.9 55.2
30-34 1666 45.4 54.6
≥35 1329 45.2 54.9
Missing 1
Maternal height, cm
<164 1345 44.1 55.9 .65
164-171 1733 45.8 54.2
>171 815 45.3 54.7
Missing 28
Maternal BMI, kg/m2
<18.5 67 53.7 46.3 .03
18.5-24.9 2362 46.2 53.9
25-29.9 963 41.5 58.5
≥30 350 47.4 52.6
Missing 179
Smoking, early pregnancy
Yes 122 45.9 54.1 .87
No 3775 45.1 54.9
Missing 24
Living with partner
Yes 3742 45.2 54.8 .64
No 131 47.3 52.7
Missing 48
Maternal country of birth
Nordic 2871 46.0 54.0 .05
Non-Nordic 876 42.2 57.8
Missing 174
In vitro fertilization
Yes 124 43.55 56.45 .73
No 3797 45.1 54.9
Infant sex
Girl 1988 44.9 55.1 .85
Boy 1933 45.2 54.8
Abbreviation: BMI, body mass index.
*P-values were derived from the chi-squared test.
TA B L E 2 Placental location according to maternal characteristics and infant sex in women with a previous cesarean delivery (n = 3921)
anterior location was not associated with an increased risk for re- tained placenta (OR 0.84, 95% CI 0.60-1.20). The results were similar when the analysis was restricted to retained placenta in combination with severe postpartum hemorrhage (OR 0.90, 95% CI 0.60-1.40 [Table 4]). Adjustments did not affect the estimates in a significant way.
In a subanalysis of women with a previous cesarean section, non-anterior placental location was further divided into fundal, lateral, and posterior location. Compared with posterior placental location, we found that both fundal and lateral placental locations were associated with a threefold increased risk of retained placenta (Table 3). However, anterior compared with posterior placental lo- cation had no association with retained placenta (OR 1.04, 95% CI
0.7 to −1.52 [Table 3]). The results were similar for retained placenta with severe postpartum hemorrhage (Table 4). Adjustments did not affect the estimates in a significant way.
4 | DISCUSSION
In this large, population-based study of women with a vaginal second delivery, we found an increased risk of retained placenta in women with a previous cesarean section, compared with women with a pre- vious vaginal delivery. However, the association between retained placenta and a previous cesarean delivery was not mediated by an- terior placental location.
Total number
Retained placenta Number (%) with outcome and placental location
Crude OR (95%
CI) aOR (95% CI)a
Placental location
Anterior 1767 55 (3.1) 0.84 (0.60-1.20) 0.94 (0.65-1.36)
Non-anterior 2154 79 (3.7) Reference Reference
Detailed placental location
Anterior 1767 55 (3.1) 1.04 (0.72-1.52) 1.24 (0.83-1.85)
Fundal 133 12 (8.6) 3.06 (1.60-5.86) 3.72 (1.92-7.21)
Lateral 110 10 (9.1) 3.24 (1.61-6.54) 4.03 (1.97-8.23)
Posterior 1905 57 (3.0) Reference Reference
Abbreviations: aOR, adjusted odds ratio; CI, confidence interval.
aAdjusted for maternal age, height, smoking in early pregnancy, and country of birth, as well as use of in vitro fertilization and sex of the infant. Only pregnancies with complete data on all covariates were included (n = 3698).
TA B L E 3 Rates and odds ratios of retained placenta according to placental location, in women with a previous cesarean delivery (n = 3921)
Total number
Retained placenta in combination with severe postpartum hemorrhage
Number (%) Crude OR (95% CI) aOR (95%
CI)a Placental location
Anterior 1767 39 (2.2) 0.91 (0.60-1.40) 1.02
(0.65-1.58)
Non-anterior 2154 53 (2.4) Reference Reference
Detailed placental location
Anterior 1767 39 (2.2) 1.14 (0.72-1.80) 1.37
(0.84-2.24)
Fundal 139 8 (5.8) 3.08 (1.41-6.76) 3.85
(1.72-8.60)
Lateral 110 7 (6.4) 3.43 (1.49-7.88) 4.38
(1.87-10.24)
Posterior 1905 37 (1.9) Reference Reference
Abbreviations: aOR, adjusted odds ratio; CI, confidence interval.
aAdjusted for maternal age, height, smoking in early pregnancy, and country of birth, as well as use of in vitro fertilization and sex of the infant. Only pregnancies with complete data on all covariates were included (n = 3698).
TA B L E 4 Rates and odds ratios of retained placenta in combination with severe postpartum hemorrhage (>1000 mL) according to placental location, in women with a previous cesarean delivery (n = 3921)
In line with previous studies, our results confirm the increased risk of retained placenta in women with a previous cesarean sec- tion.5,10-12 Some earlier case-control studies did not find an in- creased risk of retained placenta after a previous cesarean section, but those studies were small and previous cesarean section was not the main exposure.13-15
The mechanism for the increased risk of retained placenta in women with a previous cesarean section is still unknown. Different causes have been suggested, as for example a mild form of abnor- mally invasive placentation, when the placenta might be adherent to the scar in the uterine wall.5 Another suggested mechanism is in- adequate contractility of the retro-placental myometrium,12 which might be a result of a placenta covering the scar after a previous cesarean. As we did not find any association between anterior pla- cental location and retained placenta in women with a previous ce- sarean section, these mechanisms seem to be unlikely. Rather, the overall contractility in the uterus might be impaired after a previous cesarean delivery. However, we were not able to distinguish whether the placenta covered the scar at the second-trimester ultrasound scan in women with an anterior placental location, which might have diluted our findings.
When considering detailed placental location in women with a previous cesarean section, we found both fundal and lateral pla- cental locations, compared with posterior placental location, to be associated with a threefold increased risk of retained placenta.
Interestingly, this is a similar pattern to that previously shown in a study of nulliparous women from the same population-based Stockholm-Gotland Obstetric Database,9 but with generally higher rates of retained placenta for all placental locations in women with a previous cesarean section (3.4%) compared with nulliparous women (3.0%).
Hence, results from this and a previous study on nulliparous women from the same population-based database, show that the risk of retained placenta seems to be lowest in women with a pre- vious vaginal delivery (2.0%) and highest in women with a previous cesarean section (3.4%), while the risk for nulliparous women is in between (3.0%).9 These results might support the hypothesis that general contractility of the uterus is important for a normal expulsion of the placenta during the third stage of labor, and that a previous cesarean section might affect contractility. The possible underlying mechanism for the association between fundal and lateral placen- tal locations and retained placenta is however unknown. Controlled cord traction, which has been shown to reduce the risk of retained placenta during the third stage of labor, is usually recommended in settings where skilled birth attendants are available.16 The traction angle in fundal and lateral compared with anterior or posterior pla- cental locations might, however, be disadvantageous for placental release, which might explain the increased risk of manual removal of the placenta in these groups.
We found a borderline association between infant girls and re- tained placenta compared with infant boys (Table 1; OR 1.41; 95% CI 1.00-2.01). The reason for this possible association is unclear. A pos- sible explanation is the lower average birthweight in girls compared
with boys. Low birthweight and to be born small for gestational age have been associated with retained placenta.17
Moreover, we found an association between BMI and placental location, with the highest proportion of non-anterior placental loca- tion in overweight women (BMI 25.0-29.9 kg/m2). High BMI is asso- ciated with suboptimal visualization of fetal anatomy.18 One possible explanation for the association between high BMI and retained pla- centa might be that placental location is more difficult to visualize in women with high BMI, and placental locations of fundal or lateral locations might be more often misclassified in this population.
The strengths of our study include its large sample size and the population-based design, including basically all women with a vaginal second delivery from the Stockholm-Gotland region, with prospectively collected detailed information from antenatal and delivery care, including standardized information from ultrasound scans and maternal characteristics. We were able to explore many potential confounders and to confirm that the results do not depend on confounding. The study design, with only women with a vaginal second delivery, with singleton pregnancies, eliminates the poten- tial influence of multiple pregnancies. To our knowledge, this is the largest study in this field. Apart from placenta previa, the associa- tion between placental location and adverse pregnancy outcomes in women with a vaginal second delivery and a previous cesarean section has barely been studied.
The major limitation of our study is the absence of information on whether the placenta covered the scar at the second-trimester ultrasound scan. Anyway, even in the case of a possible low-lying placenta in the second trimester, over 90% of women will, due to
“placental migration”, no longer have this condition in the third tri- mester.19 However, our intention was to study placental location at the second-trimester ultrasound scan, as this is a routine scan per- formed in almost all pregnancies in Sweden and many other coun- tries, with the theoretical possibility to include ultrasound findings in pregnancy and delivery risk assessment.
Another limitation of our study is the lack of complete, detailed, and validated information about placental location. Apart from placenta previa, there is no official classification regarding placen- tal location, either nationally or internationally.20 In our study, we chose to categorize placental location into four locations, although placentas are rarely completely fundal, lateral, anterior, or posterior.
This categorization might therefore be imprecise. However, data on placental location were registered before the outcome, and any mis- classification can therefore be assumed to be non-differential.
Reporting on placental location was not mandatory on the ul- trasound form, and information in free text was not available in the database. However, in 94% of the overall study population, there was adequate information about placental location. Ultrasound data regarding placental location has not been validated. This is of course a weakness of this study—the examinations have not been performed in line with a study protocol, and placental location has not been validated. On the other hand, this can also be considered to be a strength. Placental location was determined in everyday clin- ical practice, and the generalizability of the results to the Swedish
pregnant population should thus be high, rendering high external validity. Another limitation of our study is the difficulty in estimating postpartum blood loss, but this possible source of misclassification should be non-differential.
Although this study was conducted in Sweden, the results should be generalizable to other settings, as the proportion of anterior and non-anterior placental location (fundal, lateral, and posterior) was comparable to the proportions observed in the two hitherto largest studies in this field.21,22
5 | CONCLUSION
We found an increased risk of retained placenta in women with a previous cesarean section, compared with women with a previous vaginal delivery. However, the increased risk of retained placenta in women with a previous cesarean delivery was not mediated by ante- rior placental location.
ACKNOWLEDGMENTS
We acknowledge Gunnar Petersson, database manager of the Obstetric database, for creating the data set used in this study.
CONFLIC T OF INTEREST None.
ORCID
Michaela Granfors https://orcid.org/0000-0002-2925-3779 Anna Sandström https://orcid.org/0000-0003-2879-4271 Olof Stephansson https://orcid.org/0000-0003-1528-4563
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How to cite this article: Granfors M, Sandström A, Stephansson O, Belachew J, Axelsson O, Wikström A-K.
Placental location and risk of retained placenta in women with a previous cesarean section: A population-based cohort study. Acta Obstet Gynecol Scand. 2020;99:1666–1673.
https://doi.org/10.1111/aogs.13943
