INTRODUCTION — Placenta previa refers to the presence of placental tissue that extends over the internal cervical os. Sequelae include the need for cesarean delivery, as well as the potential for severe antepartum bleeding, preterm birth, and postpartum hemorrhage.
Placenta previa should be suspected in any pregnant woman beyond 20 weeks of gestation who presents with vaginal bleeding. For women who have not had a second-trimester ultrasound examination, bleeding after 20 weeks of gestation should prompt sonographic determination of placental location before a digital vaginal examination is performed because palpation of the placenta can cause severe hemorrhage.
This topic will discuss the epidemiology, clinical features, diagnosis, morbidity, and mortality of placenta previa. The natural history of placenta, risk of bleeding, and management are reviewed separately. (See "Placenta previa: Management".)
EPIDEMIOLOGY
Prevalence — In systematic reviews, the pooled prevalence of placenta previa is approximately 4 per 1000 births but varies worldwide [1,2]. The prevalence is severalfold higher around 20 weeks of gestation (as high as 2 percent) than at birth because most previas identified early in pregnancy resolve before delivery. (See 'Natural history' below.)
Risk factors
●The major risk factors for placenta previa include [3-15]:
•Previous placenta previa – Previous placenta previa is an independent risk factor for recurrence, which occurs in 4 to 8 percent of subsequent pregnancies [16,17].
•Previous cesarean delivery – In two systematic reviews, previous cesarean delivery increased the risk for placenta previa by 47 percent [18] and 60 percent [3]. The risk increases with an increasing number of cesarean deliveries [19-21]. Prelabor cesarean delivery may increase previa risk in a subsequent delivery more than previous intrapartum cesarean delivery [13].
•Multiple gestation – The prevalence of placenta previa was 40 percent higher among twin births than among singleton births (3.9 and 2.8 per 1000 births, respectively) in one study [5]. In another study, dichorionic twin pregnancies were more likely to have a placenta previa than monochorionic twin pregnancies (odds ratio [OR] 3.3) or singleton pregnancies (OR 1.5) [22].
●Other risk factors, some of which are interdependent, include [23,24]:
•Previous uterine surgical procedure
•Increasing parity
•Increasing maternal age
•Infertility treatment
•Previous pregnancy termination
•Maternal smoking
•Maternal cocaine use
•Male fetus
•Prior uterine artery embolization
PATHOGENESIS — The pathogenesis of placenta previa is unknown. One hypothesis is that areas of suboptimally vascularized decidua in the upper uterine cavity resulting from previous surgery or multiple pregnancies promote implantation of trophoblast in, or unidirectional growth of trophoblast toward, the lower uterine cavity [1,3,25]. Another hypothesis is that a particularly large placental surface area, as in multiple gestation, increases the probability that the placenta will encroach upon or cover the cervical os.
PATHOPHYSIOLOGY OF BLEEDING — Placental bleeding is the major adverse sequelae of placenta previa. It is thought to occur when uterine contractions or gradual changes in the cervix and lower uterine segment apply shearing forces to the inelastic placental attachment site, resulting in partial detachment. Vaginal examination or coitus can also disrupt this site and cause bleeding. Bleeding is primarily maternal blood from the intervillous space, but fetal bleeding can occur if fetal vessels in the terminal villi are disrupted.
CLINICAL PRESENTATION AND COURSE
Asymptomatic finding on midtrimester ultrasound examination — The most common presentation of placenta previa is as a finding on routine ultrasound examination at approximately 16 to 20 weeks of gestation for assessment of gestational age, fetal anatomic survey, cervical length, or prenatal diagnosis. As an example, 10.6 percent of patients undergoing routine transvaginal cervical length screening have a low-lying placenta (placental edge within 2 cm of the internal os) or placenta previa [26]. However, most of these early diagnoses do not persist to term, and early diagnosis may invoke unnecessary anxiety for patients and increase costs because of follow-up ultrasound examination [26]. (See 'Natural history' below and 'Ultrasonography' below.)
Natural history — Approximately 90 percent of placenta previas identified on ultrasound examination before 20 weeks of gestation resolve before delivery [27]. Two theories have been proposed to account for this phenomenon:
●The lower uterine segment lengthens from 5 mm at 20 weeks of gestation to more than 50 mm at term [16]. This development of the lower uterine segment relocates the stationary lower edge of the placenta away from the internal os.
●Since the lower uterine segment is relatively less vascular than other parts of the myometrium, the placenta preferentially grows at its more cephalad position. This leads to progressive unidirectional growth of trophoblastic tissue toward the fundus results and an upward "migration" of the placenta away from the cervix. This phenomenon has been termed "trophotropism."
In either case, the placental edge overlying the cervix atrophies.
Predicting presence at delivery — Findings that suggest that a placenta previa will persist until delivery include lack of resolution by the third trimester, extension over the os by more than 25 mm, and posterior previa.
●Gestational age – The chances of persistence at delivery when the most recent ultrasound shows a previa are depicted in the algorithm (figure 1).
●Degree of extension over the os – The more the placenta extends over the internal os, the more likely it is to persist until delivery. Although available data are insufficient to make precise predictions, pooled data suggest that, at 18 to 24 weeks of gestation, when the placenta extends [28-33]:
•<14 mm over the internal os, the probability of placenta previa at delivery is near zero.
•≥14 mm but <25 mm over the os, the probability of placenta previa at delivery is approximately 20 percent.
•≥25 mm over the os, the probability of placenta previa at delivery ranges from 40 to 100 percent and is 100 percent at >55 mm.
In a prospective cohort study of 275 patients with placenta previa (ie, placenta overlying the internal os) not complicated by accreta, diagnosed by transvaginal ultrasound with an empty bladder at 18 to 24 weeks, and available for follow-up study, at repeat evaluation in the third trimester, only 37 (14 percent) had a persistent previa [33]. The authors noted that the pregnant patient whose placenta overlapped the internal os by less than 14 mm had a likelihood ratio (LR) of 0 for persistence, whereas those whose placenta overlapped the internal os by more than 55 mm had a LR of infinity for persistence. Those whose placentas overlayed the internal os between 14 and 55 mm had LRs ranging from 0.6 to 6.3, respectively.
In a retrospective cohort study of 319 patients with sonographic evidence of placenta previa or low-lying placentas at 32 weeks of gestation and reevaluated three weeks later, 100 percent of those with complete previa (121) and 90.6 percent (58 of 64) of those with placentas less than 10 mm from internal os failed to migrate [34]. Conversely, of the 138 patients whose placental edge was greater than 10 mm from the internal os, only 17 (12.3 percent) failed to migrate above 20 mm. (See 'Morbidity of low-lying placenta' below.)
●Anterior versus posterior position – Anterior placentas seem more likely to move away from the cervical os with advancing gestation than posterior placentas [35-38]. In a prospective study, of the 67 cases of second-trimester anterior placenta previa, 5 (7.5 percent) were still overlying the internal os and 5 (7.5 percent) were lying within 20 mm of the internal os in the third trimester [35]. Of the 114 cases of second-trimester posterior placenta previa, 18 (15.8 percent) were still overlying the internal os and 9 (7.9 percent) were lying within 20 mm of the internal os in the third trimester.
Recommendations for follow-up sonography to determine resolution are discussed separately. (See "Placenta previa: Management", section on 'Monitoring placental position'.)
Bleeding — In the second half of pregnancy, the most common symptom of placenta previa is relatively painless vaginal bleeding, which occurs in up to 90 percent of persistent cases [39]. Ten to 20 percent of women present with uterine contractions, pain, and bleeding, similar to the presentation of abruptio placentae [40,41]. (See "Placental abruption: Pathophysiology, clinical features, diagnosis, and consequences".)
In women with persistent placenta previa:
●Approximately one-third have an initial bleeding episode before 30 weeks of gestation; this group is more likely to require blood transfusions and is at greater risk of preterm delivery and perinatal mortality than women whose bleeding begins later in gestation [40-43].
●Approximately one-third become symptomatic between 30 and 36 weeks.
●Most of the remaining one-third have their first bleed after 36 weeks [40,41].
●Approximately 10 percent reach term without bleeding.
The number of antepartum bleeding episodes and need for blood transfusion have been identified as independent predictors of emergency cesarean delivery [44]. Antepartum bleeding from any cause is a risk factor for preterm labor and preterm prelabor rupture of membranes. (See "Spontaneous preterm birth: Pathogenesis", section on '#3 Decidual hemorrhage' and "Preterm prelabor rupture of membranes: Clinical manifestations and diagnosis", section on 'Risk factors'.)
Risk factors for antepartum bleeding include complete placenta previa, short cervical length, anterior placenta [24,45-47], repeated bleeding episodes, higher gravidity, and intrauterine procedure [48]. Anterior placenta previa is also associated with a larger volume of bleeding at cesarean [45,49-51].
DIAGNOSIS — Placenta previa should be suspected in any woman beyond 20 weeks of gestation who presents with vaginal bleeding. For women who have not had a second- or third-trimester ultrasound examination, antepartum bleeding should prompt sonographic determination of placental location before digital vaginal examination is performed because palpation of the placenta can cause severe hemorrhage.
Diagnostic criteria — The diagnosis of placenta previa is based on sonographic identification of echogenic homogeneous placental tissue extending over the internal cervical os on a second- or third-trimester imaging study (image 1), preferably using transvaginal ultrasound (TVUS) [52]. The distance (millimeters) that the placenta extends over the internal cervical os should be described in the diagnostic report.
Of note, when the placental edge is <20 mm from, but not over, the internal os, the placenta is labeled "low-lying" (image 2) and the distance (millimeters) between the internal cervical os and the inferior edge of the placenta is described in the diagnostic report [53].
The significance of the findings depends, in large part, on the gestational age at the time of examination. (See 'Predicting presence at delivery' above.)
The historic terms "marginal" and "partial" for characterizing a placenta previa are no longer used, as they referred to information gathered on a digital vaginal examination, which should be avoided and is no longer needed given the superiority of ultrasound diagnosis.
Ultrasonography — Transabdominal ultrasonography is the standard initial sonographic approach in most pregnant women. A subsequent TVUS examination is performed when optimal visualization of the relationship between the placenta and cervix is needed. Translabial (transperineal) ultrasound imaging is an alternative technique to TVUS as it provides excellent images of the cervix and placenta [54].
Three-dimensional ultrasound may improve accuracy over transvaginal sonography, and can be considered where available and when the information will affect clinical management [55].
Transabdominal ultrasound — Transabdominal sonographic assessment of placental location is one of the standard components of the basic obstetric ultrasound examination and, thus, a screening test for placenta previa.
Screening performance — If the distance between the edge of the placenta and the cervical os is ≤20 mm on transabdominal ultrasound, we perform transvaginal sonography to better define placental position and make the diagnosis. The overall false-positive rate of transabdominal ultrasound for diagnosis of placenta previa is up to 25 percent and varies by study design [56-58].
Technique and pitfalls — Sagittal, parasagittal, and transverse sonographic views should be obtained with the patient's bladder partially full.
Specific points that should be appreciated when performing transabdominal sonographic examination for placenta previa include:
●An over-distended bladder can compress the anterior lower uterine segment against the posterior lower uterine segment to give the appearance of a previa (image 3). The diagnosis of placenta previa should not be made without confirming placental position after the patient has emptied their bladder.
●The diagnosis of placenta previa should not be made when the lower uterine segment is contracting (which commonly occurs after a woman empties her bladder) and obscures the relationship between the placental edge and the cervical os. A contraction should be suspected when the anterior and posterior myometrium adjacent to the cervix appear more thickened than normal. Waiting 15 to 20 minutes and repeating the scan will allow time for the contraction to resolve.
●A previa can be missed near term if the fetal head is low in the pelvis since acoustic shadowing from or compression of placental tissue by the fetal skull may obscure the placental location. In these cases, the cervix may be better visualized by placing the patient in Trendelenburg position and/or gently pushing the fetal head cephalad with an abdominal hand or the transducer.
●The sonographic diagnosis of a central placenta previa is readily made since the placenta is centered over the cervix and placental tissue is imaged anterior and posterior to the cervix. Complete noncentral previas, particularly when lateral, are more difficult to confirm. Transverse views at and above the internal cervical os should facilitate an accurate diagnosis.
●A posterior placenta previa may be more difficult to visualize than an anterior placenta previa, even on TVUS.
●Bleeding can result in formation of a hematoma under and/or proximate to the placenta, which can obscure the relationship between the placental edge and the cervical os. Blood products change in echogenicity over time, thus the sonographic appearance of the bleed and placenta edge will change in as little as two to three days. If prompt diagnosis of placenta previa is clinically important in this setting, then magnetic resonance imaging can be performed to distinguish between blood products and a previa.
Transvaginal ultrasound — TVUS can be performed safely in patients with placenta previa since the optimal position of the vaginal probe for visualization of the internal os is 20 to 30 mm away from the cervix and the angle between the cervix and vaginal probe is sufficient to prevent the probe from inadvertently slipping into the cervical canal [59].
Diagnostic performance — TVUS generally provides a clearer image of the relationship between the edge of the placenta and the internal cervical os than transabdominal ultrasound. Randomized trials and prospective comparative studies have established the superior performance of TVUS over transabdominal sonography for diagnosis of placenta previa [60-62]. In one study of 100 suspected cases proximate to delivery, sensitivity, specificity, and positive and negative predictive values of TVUS for diagnosis of placenta previa at cesarean were 87.5, 98.8, 93.3, and 97.6 percent, respectively [63]. On a midtrimester ultrasound, findings suggestive of placenta previa are much less predictive of placental location at or near term, as discussed above. (See 'Asymptomatic finding on midtrimester ultrasound examination' above.)
Color Doppler is employed in previa cases in which an abnormally attached placenta (eg, placenta accreta spectrum) is suspected. It is also used to exclude vasa previa when the umbilical cord is in the lower uterine segment. (See "Placenta accreta spectrum: Clinical features, diagnosis, and potential consequences" and "Velamentous umbilical cord insertion and vasa previa".)
Imaging findings predictive of antepartum bleeding — Characteristics that appear to be predictive of antepartum bleeding include the following:
●Extension over the internal os rather than lying proximate to it [64-67].
●Thick (>10 mm) placental edge and/or angle between the basal and chorionic plates greater than 45 degrees [68,69].
●Echo-free space in the placental edge over the internal os [70].
●Cervical length ≤30 mm [69,71,72].
●Second to third trimester decrease in cervical length by >6 mm or third-trimester cervical length ≤35 mm [73,74].
Magnetic resonance imaging — Magnetic resonance imaging (MRI) is well-suited to assess placental-cervical relationships because of the differing magnetic resonance characteristics of the two tissues. However, it is not used for diagnosis of placenta previa because of the well-established safety and accuracy of TVUS, as well as the high cost and limited availability of MRI [53]. MRI is most useful for diagnosis of complicated placenta previa, such as previa-accreta spectrum [75]. (See "Placenta accreta spectrum: Clinical features, diagnosis, and potential consequences", section on 'Magnetic resonance imaging (MRI)'.)
ASSOCIATED FINDINGS
Placenta previa-accreta spectrum — When placenta previa is diagnosed, the possibility of placenta previa-accreta spectrum (PAS) should be considered. This is particularly important when the placenta lies over the area of prior hysterotomy (cesarean incision scar), and thus is most common with an anterior placenta previa. In a prospective study including 723 women with placenta previa undergoing cesarean birth, the frequency of PAS at cesarean increased with an increasing number of cesareans as follows [76]:
●First (primary) cesarean birth – PAS in 3 percent
●Second cesarean birth – PAS in 11 percent
●Third cesarean birth – PAS in 40 percent
●Fourth cesarean birth – PAS in 61 percent
●Fifth or greater cesarean birth – PAS in 67 percent
In another large series, composite maternal morbidity in women with placenta previa and zero, one, two, or three prior cesarean deliveries was 15, 23, 59, and 83 percent, respectively, and almost all of the excess composite maternal morbidity in women with a prior cesarean was related to complications associated with PAS, such as peripartum hysterectomy [77]. (See "Placenta accreta spectrum: Clinical features, diagnosis, and potential consequences", section on 'Prenatal diagnosis'.)
Other associated findings
●Malpresentation – The large volume of placenta in the lower portion of the uterine cavity predisposes the fetus to assume a noncephalic lie [60,78-80].
●Vasa previa and velamentous umbilical cord – Placenta previa is a risk factor for vasa previa and velamentous umbilical cord insertion. (See "Velamentous umbilical cord insertion and vasa previa".)
●Fetal growth restriction – An increased risk of growth restriction has been reported by multiple investigators [8,40,81-86], but not all [42,43,81,87-89], and remains controversial. If a reduction in fetal growth occurs, it is likely to be mild or due to confounding factors. (See "Fetal growth restriction: Evaluation and management".)
●Congenital anomalies – Population-based cohort studies have reported a small increase in the overall rate of congenital anomalies in pregnancies complicated by placenta previa, but no single anomaly or syndrome was associated with the disorder [8,43,90].
MORBIDITY AND MORTALITY
Maternal — Maternal morbidity from placenta previa is primarily related to antepartum and/or postpartum hemorrhage [91]. In systematic reviews, 52 percent of women with placenta previa had antepartum bleeding (95% CI 42.7-60.6 percent) [39] and 22 percent had postpartum hemorrhage (95% CI 15.8-28.7 percent) [92]. Because of antepartum and/or postpartum bleeding, women with placenta previa are more likely to receive blood transfusions and undergo postpartum hysterectomy. In a study of primary cesarean deliveries for placenta previa without placenta accreta, previa was associated with increased risk for maternal hemorrhagic morbidity (19 versus 7 percent, adjusted relative risk [aRR] 2.6, 95% CI 1.9-3.5), atony requiring uterotonics (aRR 3.1, 95% CI 2.0-4.9), red blood cell transfusions (aRR 3.8, 95% CI 2.5-5.7), and hysterectomy (aRR 5.1, 95% CI 1.5-17.3) compared with no previa at cesarean [93]. The risks of hemorrhage and postpartum hysterectomy are particularly high for women with previa-accreta spectrum. (See "Placenta accreta spectrum: Clinical features, diagnosis, and potential consequences", section on 'Consequences'.)
In women with severe hemorrhage, rapid, significant loss of intravascular volume can lead to hemodynamic instability, hypoxemia, hypoxia, organ damage, and death. In resource-abundant countries, the maternal mortality rate associated with placenta previa is less than 1 percent [94] but remains high in resource-poor countries where maternal anemia, lack of medical resources, and home births are more common [80].
Severe maternal morbidity and maternal mortality can also be a consequence of amniotic fluid embolism syndrome. Several studies have observed a strong association between placental pathology, such as placenta previa, and amniotic fluid embolism syndrome [95-97]. (See "Amniotic fluid embolism".)
Neonatal — The principal causes of neonatal morbidity and mortality are related to preterm birth [98]. In a meta-analysis of studies of placental implantation abnormalities and risk of preterm birth, compared with no placenta previa, placenta previa was associated with a three- to fivefold increase in risk of [99]:
●Preterm birth <37 weeks (44 percent, RR 5.32, 95% CI 4.39-6.45)
●Neonatal intensive care unit admission (RR 4.09, 95% CI 2.80-5.97)
●Neonatal death (RR 5.44, 95% CI 3.03-9.78)
●Perinatal death (RR 3.01, 95% CI 1.41-6.43)
In large studies, approximately 15 percent of women with placenta previa delivered before 34 weeks of gestation [84,100]. However, neonatal morbidity and mortality rates in pregnancies complicated by placenta previa have fallen over the past few decades because of improvements in obstetric management (eg, antenatal corticosteroids), the liberal use of planned late preterm cesarean delivery, and improved neonatal care.
Neonatal anemia is also increased in pregnancies with placenta previa [101,102].
The risk for adverse neonatal outcome appears to be higher in patients with recurrent bleeding [48].
Morbidity of low-lying placenta — The morbidity of low-lying placenta (placental edge is <20 mm from, but not over, the internal os) is less than that for placenta previa and decreases as the distance between the placental edge and internal cervical os increases. A study that compared pregnancy outcomes of 53 women with placental edge 1 to 10 mm versus 11 to 20 mm from the internal cervical os reported higher rates of the following [103]:
●Antepartum hemorrhage – 29 versus 3 percent
●Postpartum hemorrhage – 21 versus 10 percent
●Preterm birth – 29 versus 3 percent
●Cesarean delivery – 75 versus 31 percent
RECURRENCE — Placenta previa recurs in 4 to 8 percent of subsequent pregnancies [16,17].
MANAGEMENT — (See "Placenta previa: Management".)
SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Obstetric hemorrhage".)
INFORMATION FOR PATIENTS — UpToDate offers two types of patient education materials, "The Basics" and "Beyond the Basics." The Basics patient education pieces are written in plain language, at the 5th to 6th grade reading level, and they answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials. Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are written at the 10th to 12th grade reading level and are best for patients who want in-depth information and are comfortable with some medical jargon.
Here are the patient education articles that are relevant to this topic. We encourage you to print or e-mail these topics to your patients. (You can also locate patient education articles on a variety of subjects by searching on "patient info" and the keyword(s) of interest.)
●Basics topics (see "Patient education: Placenta previa (The Basics)")
SUMMARY AND RECOMMENDATIONS
●Prevalence, presentation, natural history – One to 6 percent of pregnant women display sonographic evidence of a placenta previa between 10 and 20 weeks of gestation when they undergo routine obstetric ultrasound examination. The majority of these women are asymptomatic, and >90 percent of these early cases resolve (figure 1). The likelihood of placenta previa at delivery is high when the previa persists into the third trimester and extends over the internal cervical os by ≥25 mm. (See 'Asymptomatic finding on midtrimester ultrasound examination' above.)
●Clinical manifestations – The characteristic symptom of placenta previa is painless vaginal bleeding, which occurs in up 90 percent of persistent cases. Ten to 20 percent of symptomatic women present with both uterine contractions and bleeding, similar to the presentation of abruptio placenta. In approximately one-third of pregnancies with persistent previa, the initial bleeding episode occurs prior to 30 weeks of gestation. (See 'Bleeding' above.)
●Risk factors – Previous placenta previa, previous cesarean deliveries, and multiple gestation are major risk factors for placenta previa. (See 'Epidemiology' above.)
●Diagnosis
•Placenta previa should be suspected in any woman beyond 20 weeks of gestation who presents with vaginal bleeding. For women who have not had a second or third trimester ultrasound examination, antepartum bleeding should prompt sonographic determination of placental location before digital vaginal examination is performed because palpation of the placenta can cause severe hemorrhage. (See 'Diagnosis' above.)
•The diagnosis of placenta previa is based on identification of placental tissue over the internal cervical os on an imaging study (image 1). Transvaginal sonography should be performed to confirm a diagnosis made on transabdominal imaging. The distance (millimeters) that the placenta extends over the internal cervical os should be described in the diagnostic report. (See 'Diagnosis' above.)
•When placenta previa is diagnosed, the possibility of placenta previa-accreta spectrum should be considered and excluded, especially in women with a previous cesarean delivery. (See 'Placenta previa-accreta spectrum' above.)
●Associated conditions – Other conditions that may be associated with placenta previa include malpresentation, preterm labor or preterm prelabor rupture of the membranes, vasa previa, and velamentous insertion of the umbilical cord. (See 'Other associated findings' above.)
●Maternal and neonatal morbidity – Maternal morbidity from placenta previa is primarily related to antepartum and/or postpartum hemorrhage, which can be life-threatening. The principal causes of neonatal morbidity and mortality are related to preterm birth. (See 'Morbidity and mortality' above.)
●Recurrence – Placenta previa recurs in 4 to 8 percent of subsequent pregnancies. (See 'Recurrence' above.)
1 : Etiology and risk factors for placenta previa: an overview and meta-analysis of observational studies.
2 : Prevalence of placenta praevia by world region: a systematic review and meta-analysis.
3 : The association of placenta previa with history of cesarean delivery and abortion: a metaanalysis.
4 : The association of placenta previa with history of cesarean delivery and abortion: a metaanalysis.
5 : Placenta previa in singleton and twin births in the United States, 1989 through 1998: a comparison of risk factor profiles and associated conditions.
6 : Placenta previa: preponderance of male sex at birth.
7 : Placenta previa: its relationship with race and the country of origin among Asian women.
8 : Critical analysis of risk factors and outcome of placenta previa.
9 : The epidemiology of placenta previa in the United States, 1979 through 1987.
10 : The association between maternal cocaine use and placenta previa.
11 : Obstetric history and the risk of placenta previa.
12 : Risk of placenta previa in second birth after first birth cesarean section: a population-based study and meta-analysis.
13 : Previous prelabor or intrapartum cesarean delivery and risk of placenta previa.
14 : The association of placenta previa and assisted reproductive techniques: a meta-analysis.
15 : Smoking and placenta previa: a meta-analysis.
16 : Placenta previa.
17 : Trends and recurrence of placenta praevia: a population-based study.
18 : Cesarean section and placental disorders in subsequent pregnancies--a meta-analysis.
19 : Placenta previa/accreta and prior cesarean section.
20 : Previous cesarean delivery and risks of placenta previa and placental abruption.
21 : The likelihood of placenta previa with greater number of cesarean deliveries and higher parity.
22 : Natural history of placenta previa in twins.
23 : Maternal risk factors associated with persistent placenta previa.
24 : Maternal and Neonatal Outcomes Resulting from Antepartum Hemorrhage in Women with Placenta Previa and Its Associated Risk Factors: A Single-Center Retrospective Study.
25 : Aetiological factors in placenta praevia--a case controlled study.
26 : Universal transvaginal cervical length screening during pregnancy increases the diagnostic incidence of low-lying placenta and placenta previa.
27 : Placenta previa, placenta accreta, and vasa previa.
28 : Placenta previa: antepartum conservative management, inpatient versus outpatient.
29 : The relevance of placental location at 20-23 gestational weeks for prediction of placenta previa at delivery: evaluation of 8650 cases.
30 : Transvaginal ultrasonography at 18-23 weeks in predicting placenta previa at delivery.
31 : Clinical significance of placenta previa detected at early routine transvaginal scan.
32 : Diagnosis of low-lying placenta: can migration in the third trimester predict outcome?
33 : Which second trimester placenta previa remains a placenta previa in the third trimester: A prospective cohort study.
34 : Diagnosis and management of placenta previa and low placental implantation.
35 : Follow-up ultrasound in second-trimester low-positioned anterior and posterior placentae: prospective cohort study.
36 : Difference in migration of placenta according to the location and type of placenta previa.
37 : Resolution of a Low-Lying Placenta and Placenta Previa Diagnosed at the Midtrimester Anatomy Scan.
38 : Sonographic assessment of placental migration in second trimester low lying placenta.
39 : Prevalence of antepartum hemorrhage in women with placenta previa: a systematic review and meta-analysis.
40 : The conservative aggressive management of placenta previa.
41 : Placenta previa: aggressive expectant management.
42 : Maternal and perinatal morbidity resulting from placenta previa.
43 : Neonatal outcomes with placenta previa.
44 : Predictors for Emergency Cesarean Delivery in Women with Placenta Previa.
45 : Effect of site of placentation on pregnancy outcomes in patients with placenta previa.
46 : Type and location of placenta previa affect preterm delivery risk related to antepartum hemorrhage.
47 : Cervical length should be measured for women with placenta previa: cohort study.
48 : Maternal and neonatal outcomes of repeated antepartum bleeding in 493 placenta previa cases: a retrospective study.
49 : Does previa location matter? Surgical morbidity associated with location of a placenta previa.
50 : Anterior placentation as a risk factor for massive hemorrhage during cesarean section in patients with placenta previa.
51 : Maternal outcomes according to placental position in placental previa.
52 : Fetal imaging: executive summary of a joint Eunice Kennedy Shriver National Institute of Child Health and Human Development, Society for Maternal-Fetal Medicine, American Institute of Ultrasound in Medicine, American College of Obstetricians and Gynecologists, American College of Radiology, Society for Pediatric Radiology, and Society of Radiologists in Ultrasound Fetal Imaging workshop.
53 : Role of imaging in second and third trimester bleeding. American College of Radiology. ACR Appropriateness Criteria.
54 : Translabial ultrasonography and placenta previa: does measurement of the os-placenta distance predict outcome?
55 : Three-dimensional transvaginal sonography in third-trimester evaluation of placenta previa.
56 : Early identification of placenta praevia.
57 : Diagnosis and management of placenta previa.
58 : Transabdominal ultrasonography as a screening test for second-trimester placenta previa.
59 : Confirming the safety of transvaginal sonography in patients suspected of placenta previa.
60 : Transvaginal and transabdominal ultrasound for the diagnosis of placenta praevia.
61 : Transvaginal ultrasonography for all placentas that appear to be low-lying or over the internal cervical os.
62 : Transvaginal ultrasound: does it help in the diagnosis of placenta previa?
63 : Accuracy and safety of transvaginal sonographic placental localization.
64 : Complete versus incomplete placenta previa and obstetric outcome.
65 : Risk factors for peripartum blood transfusion in women with placenta previa: a retrospective analysis.
66 : Placenta previa: does its type affect pregnancy outcome?
67 : Risk factors and pregnancy outcome in different types of placenta previa.
68 : Third-trimester transvaginal ultrasonography in placenta previa: does the shape of the lower placental edge predict clinical outcome?
69 : Does cervical length and the lower placental edge thickness measurement correlates with clinical outcome in cases of complete placenta previa?
70 : Anticipation of uterine bleeding in placenta previa based on vaginal sonographic evaluation.
71 : Cervical length and risk of antepartum bleeding in women with complete placenta previa.
72 : Ultrasonographic cervical length and risk of hemorrhage in pregnancies with placenta previa.
73 : Consecutive cervical length measurements as a predictor of preterm cesarean section in complete placenta previa.
74 : Serial Change in Cervical Length for the Prediction of Emergency Cesarean Section in Placenta Previa.
75 : Accuracy of ultrasonography and magnetic resonance imaging in the diagnosis of placenta accreta.
76 : Maternal morbidity associated with multiple repeat cesarean deliveries.
77 : Pregnancy outcomes for women with placenta previa in relation to the number of prior cesarean deliveries.
78 : Placenta previa: obstetric risk factors and pregnancy outcome.
79 : Incidence and contribution of predisposing factors to transverse lie presentation.
80 : Placenta praevia: maternal morbidity and place of birth.
81 : Characteristics of patients with placenta previa and results of "expectant management".
82 : Fetal growth and placental function in patients with placenta praevia.
83 : The epidemiology and clinical history of asymptomatic midtrimester placenta previa.
84 : Relationship among placenta previa, fetal growth restriction, and preterm delivery: a population-based study.
85 : Placenta previa and the risk of delivering a small-for-gestational-age newborn.
86 : The effect of placenta previa on fetal growth and pregnancy outcome, in correlation with placental pathology.
87 : Early placenta previa and delivery outcome.
88 : Effect of placenta previa on fetal growth.
89 : A Danish national cohort study on neonatal outcome in singleton pregnancies with placenta previa.
90 : Placenta previa and risk of major congenital malformations among singleton births in Finland.
91 : Maternal complications with placenta previa.
92 : The Incidence of Postpartum Hemorrhage in Pregnant Women with Placenta Previa: A Systematic Review and Meta-Analysis.
93 : Placenta previa and maternal hemorrhagic morbidity.
94 : Placenta previa and maternal hemorrhagic morbidity.
95 : Incidence and risk factors of amniotic fluid embolisms: a population-based study on 3 million births in the United States.
96 : Amniotic fluid embolism: antepartum, intrapartum and demographic factors.
97 : Amniotic fluid embolism-risk factors, maternal and neonatal outcomes.
98 : Placenta previa: neonatal death after live births in the United States.
99 : Placental implantation abnormalities and risk of preterm delivery: a systematic review and metaanalysis.
100 : Placenta previa and the risk of preterm delivery.
101 : A comparative study of neonatal outcomes in placenta previa versus cesarean for other indication at term.
102 : Risk factors of neonatal anemia in placenta previa.
103 : Placenta previa: distance to internal os and mode of delivery.
