Trial of Labor Compared With Elective Cesarean Delivery for Low-Lying Placenta : Obstetrics & Gynecology

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Contents: Original Research

Trial of Labor Compared With Elective Cesarean Delivery for Low-Lying Placenta

Froeliger, Alizée MD, MPH; Madar, Hugo MD, MPH; Jeanneteau, Pauline MD; Ruiz, Vanessa MD; Le Lous, Maela MD; Perrotin, Franck MD, PhD; Winer, Norbert MD, PhD; Dreyfus, Michel MD, PhD; Merviel, Philippe MD, PhD; Mattuizzi, Aurélien MD; Jauniaux, Eric MD, PhD; Sentilhes, Loïc MD, PhD

Author Information
doi: 10.1097/AOG.0000000000004890

Low-lying placenta, defined as a distance between the cervical os and the placenta (internal os distance) of 20 mm or less is associated with maternal and neonatal morbidity and mortality1–4 and increases the risks of antepartum, intrapartum, and postpartum hemorrhage (PPH), and blood transfusion, hysterectomy, sepsis, and thrombophlebitis.5 Rising numbers of cesarean deliveries6–8 and of pregnancies due to medically assisted reproduction have increased the prevalence of low-lying placenta over the past two decades.6,9,10 The combined prevalence of both placenta previa (defined as the placenta lying directly over the internal os) and low-lying placenta in the literature varies widely and is estimated at around 0.5% of pregnancies at term.3,7

All national guidelines and expert opinions on the management of placenta previa recommend delivery by elective cesarean between 36 and 38 weeks of gestation in women with uncomplicated placenta previa.2–5,11 A broad consensus also proposes trial of labor at term for women with an internal os distance greater than 20 mm without other contraindications. By contrast, for women with a third-trimester asymptomatic low-lying placenta,1 mode of delivery remains controversial. No guidelines are available to aid physicians in the United States with this decision.3,5 Practices are likely similar to those in Canada,4 where elective cesarean delivery is recommended for an internal os distance of 10 mm or less due to the high hemorrhage risk; individualized management is offered for a distance of 11–20 mm from the internal os, without antenatal complications. In the United Kingdom2 and France,11 the mode of delivery for women with an asymptomatic third-trimester low-lying placenta depends on their clinical background and preferences, supplemented by transvaginal ultrasound imaging of the placental position.

Relatively few studies have examined perinatal outcomes in women diagnosed with low-lying placenta at term.12–20 None offer data on antenatal symptoms or the indication for planned mode of delivery. Furthermore, because most protocols failed to take the intended treatment into account, they likely introduced selection biases.12–17,19,20 A randomized controlled trial might resolve this question, but recruiting difficulties and ethical concerns undoubtedly make it impossible.

The aim of our study was to evaluate and compare maternal and neonatal morbidity of women with low-lying placenta by their planned mode of delivery and stratified by the internal os distance at the last predelivery ultrasound examination.


Six tertiary maternity hospitals in France participated in this retrospective multicenter study of births from 2007 through 2012. Each hospital searched its database for all consecutive case files with one of the following International Classification of Diseases, Tenth Revision codes O44 and O43.2. These codes correspond to placenta previa and low-lying placenta, with or without hemorrhage, and to placenta accreta spectrum. Two independent investigators (P.J. and V.R.) first reviewed each case to select only those with low-lying placenta to avoid misclassification. Women were not included in the study if their medical files were incomplete or contained a classification error or if the delivery took place outside of a participating center. Each medical chart was then reviewed. Women with singleton or multiple pregnancies who were diagnosed with low-lying placenta (internal os distance less than 20 mm at the last predelivery transvaginal ultrasound examination) and gave birth at or after 35 weeks of gestation were included. Exclusion criteria included placenta previa, antenatally suspected placenta accreta spectrum, and termination of pregnancy. The inclusion and diagnostic criteria were the same for all six centers.

We collected maternal baseline clinical characteristics, course of labor, mode of delivery, PPH, and maternal and neonatal outcomes. We also sought to retrieve variables that might have influenced the choice of planned mode of delivery such as characteristics of the pregnancy including antepartum hemorrhage episodes.

In France, in addition the last mandatory ultrasound examination at 32 weeks of gestation, the report of which must specify placental location, another ultrasound examination is recommended for both placenta previa and low-lying placenta at 36 weeks to determine the internal os distance and, therefore, the planned mode of delivery.11 The Research Ethics Committee at Angers University Hospital Center (ref. 2013/50) approved this study.

The primary objective of the study was severe primary PPH, defined as blood loss greater than 1,000 mL within 24 hours after delivery,21,22 measured with a collection bag in vaginal births and with graduated drapes, suction canister or by weighing in cesarean deliveries.11,23–25 This cumulative endpoint was a summation of all blood loss measurements including intrapartum blood loss.11,22Intrapartum hemorrhage was defined as blood loss that required emergency cesarean delivery.5,11,26 The main secondary endpoint was severe maternal morbidity (SMM), as previously described24,27–29 and defined by any of the following: uterine artery embolization or emergency surgery to control PPH, transfusion of more than 5 units of packed red blood cells, transfer to the intensive care unit, thromboembolic events, and death. Another secondary endpoint was severe neonatal morbidity including: 5-minute Apgar score less than 7, umbilical artery pH less than 7.10, need for resuscitation or intubation with transfer to the neonatal intensive care unit, and neonatal death.30

The exposure was the planned mode of delivery. Each medical chart was independently reviewed by two independent investigators (P.J. and V.R.) to ensure the accuracy of the planned mode of delivery and other data (described above). Trial of labor was defined as a planned trial of labor confirmed by medical records, regardless of ultimate mode of delivery—a successful vaginal delivery or an emergency cesarean delivery performed before or during labor for severe intrapartum bleeding, abnormal fetal heart rate, or failure to progress. Elective cesarean delivery was defined as a planned cesarean delivery, recorded in the medical records, and performed before labor regardless of the indication or during labor for women starting labor before the planned cesarean delivery date in the medical file. Women with a history of two or more cesarean deliveries had elective cesarean delivery, in accordance with French guidelines.31

Analyses used STATA 15. Continuous variables were described by their medians and interquartile ranges and compared between groups by a Kruskal-Wallis test. Categorical variables were described by proportions and compared with χ2 or Fisher exact tests, as appropriate.

To control for confounding factors that might have influenced both the choice of the planned mode of delivery and the primary endpoint, we began the analysis with logistic regression models with multiple adjustments to estimate crude and adjusted odds ratios (aORs) and their 95% CIs. Variables included in the multivariable analysis were chosen from the literature and included other potential confounders. Because practitioners' decisions about planned mode of delivery are probably guided by characteristics of the woman and her pregnancy rather than by chance, we performed a second analysis using a propensity score to limit potential indication bias.32,33 An inverse probability of treatment weighting based on a propensity score was used to control for factors potentially influencing both management and the primary outcome. The propensity score was defined as each woman's probability of attempting labor, based on her individual characteristics, and was estimated with a multivariable logistic regression model including the following covariates: maternal age, prepregnancy body mass index (BMI, calculated as weight in kilograms divided by height in meters squared), nulliparity, previous cesarean delivery, recurrent episodes of antepartum hemorrhage, anterior placental location, and distance between the cervical os and the placenta. We assigned women who were planning a trial of labor a weight of 1/(propensity score) and those scheduled for elective cesarean delivery a weight of 1/(1−propensity score). Standardized differences were used to assess covariate balance.

A sensitivity analysis was performed with multiple imputation of missing data. The proportion of women with missing data for any covariate included in the main multivariable model ranged from 1% to 8%. We performed multiple imputation chained equations according to Rubin’s rules for those missing data (10 data sets imputed).34 A separate analysis was performed by internal os distance (ie, 1–10 mm and 11–20 mm). P<.05 was considered significant.


During the study period, the six university hospital centers recorded 128,233 births, including 1,089 with International Classification of Diseases, Tenth Revision codes O44 and O43.2. Figure 1 presents the case selection flowchart; 171 cases could be analyzed: 70 (40.9%) in the trial-of-labor group and 101 (59.1%) in the elective cesarean delivery group.

Fig. 1.:
Study flowchart. *Low-lying placenta was defined as a distance between the placental edge and the cervical os less than 20 mm at the last predelivery transvaginal ultrasonogram; placenta previa was defined as the placenta lying directly over the cervical os.1 ICD-10, International Classification of Diseases, Tenth Revision.

Table 1 summarizes maternal and pregnancy characteristics by planned mode of delivery. Women in the trial-of-labor group had significantly lower rates of previous cesarean delivery (P=.01) and higher rates of both antepartum hemorrhage (P<.01) and hospitalization for it (P<.01). Their recurrent bleeding episodes were less frequent (P=.01). Moreover, their median [interquartile range] interval between delivery and the last ultrasound examination was shorter (4 days [0–16] vs 10.5 days [3–17], P<.01), and their median [interquartile range] internal os distance was longer (13.5 mm [9–17] vs 9 mm [0–13], P<.01) than among women with elective cesarean deliveries.

Table 1.:
Maternal Characteristics of the Study Population by Planned Mode of Delivery (N=171)

Table 2 presents maternal and neonatal outcomes by planned mode of delivery. The rate of severe PPH (blood loss greater than 1,000 mL) was similar in both groups, 22.9% (95% CI 13.7–34.4) for the trial-of-labor group compared with 23.0% (95% CI 15.2–32.5) for the cesarean delivery cohort (P=.98). The vaginal delivery rate in those planning labor was 38.6% (27/70). Moreover, the multivariable logistic regression analysis showed that trial of labor was not significantly (aOR 1.42, 95% CI 0.62–3.24, P=.39) associated with a higher rate of severe PPH after adjustment for maternal age, prepregnancy BMI, nulliparity, previous cesarean delivery, and internal os distance (Appendix 1, available online at Results were similar for the multivariable analysis after multiple imputation (aOR 1.39, 95% CI 0.65–2.84, P=.46) and for the propensity score-weighted model (aOR 1.34, 95% CI 0.53–3.38, P=.53) (Fig. 2). Differences in women's characteristics at baseline were well-balanced between the groups after propensity score weighting, with all standardized differences less than 10%, except for the variable of prepregnancy BMI 30 or higher; its standardized difference was 11% after propensity score weighting (Appendices 2 and 3, available online at

Table 2.:
Maternal and Neonatal Outcomes by Planned Mode of Delivery (N=171)
Fig. 2.:
Risk of severe postpartum hemorrhage in the trial-of-labor group (reference: elective cesarean delivery) for the main analysis, including all deliveries at and after 35 weeks of gestation (N=171). OR, odds ratio.

The rate of SMM did not differ significantly between the trial-of-labor and elective cesarean delivery groups (2.9% [2/70] vs 2.0% [2/101], respectively, P=.71), nor did that of severe neonatal morbidity (12.9% [9/70] vs 9.9% [10/101], respectively, P=.55) (Table 2).

Table 3 compares the perinatal outcomes by internal os distance. For women with an internal os distance of 11–20 mm who underwent planned trial of labor, the vaginal delivery rate was 50.0% (19/38) compared with 18.5% (5/27) in the 1–10 mm subgroup (P=.01). The rates of emergency cesarean delivery for bleeding before or during labor were 27.0% (10/37) and 50.0% (13/26) (P=.06), respectively. Neither maternal nor perinatal outcomes differed significantly between the groups. Notably, the rates of severe PPH (blood loss greater than 1,000 mL) were 18.5% (5/27) for the trial-of-labor group and 28.8% (19/67) for the cesarean delivery cohort in the 1–10 mm subgroup (P=.30), and 26.3% (10/38) and 12.9% (4/31), respectively, in the 11–20 mm subgroup (P=.17).

Table 3.:
Maternal and Neonatal Outcomes by Planned Mode of Delivery for Women With Cervix-To-Placenta Distances of 1–10 mm and 11–20 mm (n=163)*


Our results indicate that almost 40% of women with low-lying placenta at or near term who attempt labor have a successful vaginal birth with no increase in their risk of maternal or perinatal complications, compared with women who undergo elective cesarean delivery. In particular, trial of labor did not result in a higher rate of severe PPH after controlling for confounding by indication with a propensity score analysis. An internal os distance of 11–20 mm at the last predelivery transvaginal ultrasound examination, compared with 1–10 mm, increased the chance of vaginal birth.

Our findings are similar to those of the only study prospective comparing maternal and neonatal outcomes according to planned mode of delivery in women with low-lying placenta.18 It was much smaller, including only 18 women with low-lying placentas diagnosed at 36 weeks of gestation as well as six with an internal os distance exceeding 20 mm at delivery. The authors did not control for the selection and indication biases, and the women's preference determined the mode of delivery.

Our results describing the success of vaginal delivery according to internal os distance are also consistent with those of a systematic review and meta-analysis by Jansen et al of 10 studies, including seven retrospective cohort studies intended to evaluate the likelihood of vaginal delivery in women with low-lying placenta. Among the 478 women in this meta-analysis, vaginal delivery was successful in 43% with an internal os distance of 0–10 mm and in 85% with a distance of 11–20 mm. The authors further stated that PPH did not differ significantly by internal os distance in either cohort.35 Moreover, the author emphasized that only three studies13,14,18 compared blood loss by mode of delivery and found inconsistent results35: one study found no difference,14 and two others found significantly more intrapartum hemorrhages and blood transfusions in women with cesarean delivery compared with vaginal deliveries.13,18

This meta-analysis nonetheless has important limitations. First, most of the studies included in this review used a methodology that did not take the intended treatment into account, and Jansen et al provide no comparison of maternal outcomes by planned mode of delivery (ie, cesarean delivery or trial of labor). Although they report that planned cesarean deliveries were chosen in 30–60% of cases, this might have been due to unfavorable conditions for attempting labor (short internal os distance, for example), which might have resulted in planned cesarean deliveries for the less stable patients and, thus, overestimation of the vaginal delivery success rate.35 Simultaneously, this failure to take the intended treatment into account is likely to have introduced a selection bias by allocating to the trial-of-labor group women who did not have elective cesarean delivery because of their favorable prognosis. Moreover, their inclusion of women who underwent emergency cesarean delivery during labor for intrapartum hemorrhage after attempted labor to the cesarean delivery group would also have introduced bias by worsening the maternal prognosis in this group. This may explain why two of the three studies that assessed blood loss by actual mode of delivery found significantly higher blood loss and transfusion rates in the cesarean delivery than in the vaginal delivery group.13,18 Furthermore, among the studies included in this meta-analysis reporting blood loss by internal os distance or actual rather than planned mode of delivery, only one provided information about how blood loss was assessed (with a collector bag),18 three studies defined PPH using different cutoffs for vaginal and cesarean deliveries,16,19,21 and one did not define PPH at all.20 All of these points limit the robustness of the results reported by Jansen et al.35

Our data, analyzed using propensity score methodology to limit indication bias, support a policy of trial of labor for women diagnosed with low-lying placenta presenting around term with an internal os distance of 11–20 mm. Attempting labor is also a possible option for women when this distance is 1–10 mm as the incidence of blood-loss related maternal outcomes, including severe PPH, did not differ between the trial-of-labor and elective cesarean delivery groups. Avoiding unnecessary cesarean deliveries in women with low-lying placenta is crucial to limiting the occurrence in this population's subsequent pregnancies of low-lying placenta, placenta previa, vasa previa, and placenta accreta spectrum. Nevertheless, shared decision-making should emphasize that the need for emergency cesarean delivery during labor was considerably higher in women with the shortest internal os distance and reached about 80% in this group.

Transvaginal ultrasonography has become essential in the diagnosis, follow-up, and management of women with low-lying placenta. Most studies, including ours, have focused on the internal os distance. Small, observational, and retrospective studies have suggested that women with low-lying placenta are more likely to need cesarean delivery when the placental edge is thicker (more than 10 mm)15,36 or contains a sponge-like echo37 or marginal "sinus."18 These additional ultrasound features are poorly defined and not routinely assessed in current practice. Their use to advise women with low-lying placenta about mode of delivery requires further research.

The main strengths of our study were 1) comparison of trial of labor with elective cesarean delivery rather than comparison of vaginal delivery with cesarean delivery, because the latter comparison is obviously biased in favor of the vaginal delivery group; and 2) use of propensity score analyses to ensure that the study groups were comparable and to minimize the effect of uncontrolled confounders and especially indication biases linked to mode of delivery.

Second, for our primary analysis, only births at or after 35 weeks of gestation were included to limit confounding, as no perinatal center had elective cesarean delivery policy before this gestational age. Thus, before 35 weeks of gestation, it was likely that trial of labor was allowed only in the most favorable conditions (preterm labor without bleeding), and elective cesarean delivery performed in the poorest conditions (severe antenatal bleeding).

The primary limitation of our cohort study lies in its retrospective design. Nonetheless, all data for every case were collected according to a defined protocol. Although eligible cases might have been missed, the combined prevalence of low-lying and placenta previa observed in our study (0.56%) is consistent with rates reported in the literature.3,7

Second, unmeasured confounders may persist despite our use of cautious statistical approaches — sensitivity, multivariable, and propensity score analyses. A randomized controlled trial would undeniably be the best study design for comparing the effects of planned mode of delivery, but its feasibility seems doubtful. Third, the rare occurrence of SMM, such as second-line therapies to control PPH (pelvic arterial embolization or surgical therapies), admission to the intensive care unit, thromboembolic events, and maternal death limited our statistical power to detect potentially clinically meaningful differences between planned modes of delivery. In addition, considering the small difference in PPH rates between the trial-of-labor and elective cesarean delivery groups (22.9% vs 23.0%, respectively), we acknowledge that our study is underpowered to confirm an absence of difference in maternal adverse outcomes. Nonetheless, a post hoc analysis determined that with a sample size of 171 patients and 23% of deliveries complicated by severe PPH in the unexposed (elective cesarean delivery) group, the study would have had a power of 80% and an alpha risk of 0.05, able to detect an odds ratio greater than 2.5 in the univariate logistic regression. It would also have been able to detect an odds ratio greater than 2.5 in the univariate logistic regression for the risk of PPH greater than 500 mL. Lastly, because our data are a decade old, we cannot exclude the possibility that practices of antenatal imaging and labor management have changed in relevant ways.

Our results support a policy of offering trial of labor to women with low-lying placenta at or after 35 weeks of gestation and a distance of 11–20 mm between the placental edge and the internal os on ultrasonography. Although an internal os distance of 1–10 mm does not increase the incidence of severe PPH or SMM, it strongly reduces the likelihood of successful vaginal delivery, from 50% in individuals with a distance of 11–20 mm to 18% in those with a distance of 1–10 mm.


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