INTRODUCTION — Placenta previa refers to the presence of placental tissue that extends over the internal cervical os. Sequelae include the need for cesarean birth, and the potential for severe antepartum bleeding, preterm birth, and postpartum hemorrhage.
Placenta previa should be suspected in any pregnant person beyond 20 weeks of gestation who presents with vaginal bleeding. For patients 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, and potential morbidity and mortality of placenta previa. Management is reviewed separately. (See "Placenta previa: Management".)
Prevalence — In systematic reviews, the pooled prevalence of placenta previa is 4 to 5 per 1000 births but varies worldwide; the reasons for this variation are unclear [1,2]. The prevalence is much higher at 20 weeks of gestation than at birth because most cases identified early in pregnancy resolve before delivery. In one study, 11 percent of patients undergoing routine transvaginal cervical length screening at or near this gestational age had a previa or placental edge within 2 cm of the internal os . (See 'Natural history' below.)
●Major risk factors [4-16]:
•Previous placenta previa – Placenta previa recurs in 4 to 8 percent of subsequent pregnancies [17,18].
•Previous cesarean birth – In two systematic reviews, previous cesarean birth increased the risk for placenta previa by 47 percent  and 60 percent . The risk increases with an increasing number of cesarean births [20-22]. Prelabor cesarean birth may increase previa risk in a subsequent pregnancy more than previous intrapartum cesarean birth .
•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 . 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) .
●Other risk factors, some of which are interdependent, include [24-28]:
•Previous uterine surgical procedure
•Increasing maternal age
•Maternal cocaine use
•Prior uterine artery embolization
•Abortion, either spontaneous or induced
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 multiparity promote implantation of trophoblast in, or unidirectional growth of trophoblast toward, the lower uterine cavity [1,4,29]. Another hypothesis is that a particularly large placental surface area, as in multiple gestation, increases the chances 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 initial presentation of placenta previa is as a finding on the routine ultrasound examination performed at approximately 18 to 20 weeks of gestation for confirmation of gestational age, fetal anatomic survey, measurement of cervical length, placental location, and/or prenatal diagnosis. (See "Overview of ultrasound examination in obstetrics and gynecology", section on 'Standard examination'.)
Natural history — Approximately 90 percent of placenta previas identified on ultrasound examination at 18 to 20 weeks of gestation resolve before delivery at term . 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 . 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, resulting in upward "migration" of the placenta away from the cervix, a phenomenon 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 term 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 shown 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. Available data are insufficient to make precise predictions, but the following observations helpful for counseling.
At 18 to 24 weeks of gestation, pooled data suggest that when the placenta extends [31-36]:
•<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 retrospective cohort study of 319 patients with a placenta previa or low-lying placentas at 32 weeks of gestation and reevaluated three weeks later, 100 percent of previas (placental edge covering the internal os), 91 percent of low-lying placentas with the edge <10 mm from the internal os, and 12 percent of low lying placentas with the edge >10 mm form the internal os failed to resolve (ie, migrate to a lower edge position >20 mm from the internal os) by 35 weeks .
●Anterior versus posterior position – Anterior previas appear to be more likely to resolve with advancing gestation than posterior previas [38-41].
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 painless vaginal bleeding, which occurs in up to 90 percent of persistent cases . Ten to 20 percent of patients present with uterine contractions, pain, and bleeding, similar to the presentation of patients with placental abruption [43,44]. (See "Acute placental abruption: Pathophysiology, clinical features, diagnosis, and consequences".)
Among patients 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 birth and perinatal mortality than those whose bleeding begins later in gestation [43-46].
●Approximately one-third become symptomatic between 30 and 36 weeks.
●Most of the remaining one-third have their first bleed after 36 weeks [43,44].
●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 birth . 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 sonographic findings such as short cervical length and anterior placenta, and historical characteristics such as high gravidity (5 or more pregnancies) and more than three prior intrauterine procedure [25,48-50]. (See 'TVUS findings predictive of antepartum bleeding' below.)
DIAGNOSIS — Placenta previa should be suspected in any patients beyond 20 weeks of gestation who presents with vaginal bleeding. For patients 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) . The distance (millimeters) that the placenta extends over the internal cervical os should be described in the diagnostic report.
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 [52,53].
The significance of the findings depends, in large part, on the gestational age at the time of examination. Because many previas detected at 20 weeks resolve before term, an early gestational diagnosis can be considered provisional. (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 pregnancy. 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 . Three-dimensional ultrasound may improve accuracy over TVUS, and can be considered where available and when the information will affect clinical management .
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.
Diagnostic performance — The overall accuracy of third-trimester transabdominal ultrasound for diagnosis of placenta previa was 89 percent, slightly less than the 94.5 percent accuracy with TVUS in a study in which all participants were evaluated with both modalities but by different sonographers for the sequential examinations . For this reason, if the distance between the edge of the placenta and the cervical os is ≤20 mm on transabdominal ultrasound, we perform TVUS to better define placental position and confirm or exclude the diagnosis. Routine midtrimester TVUS screening for placenta previa in all pregnancies is not recommended since the increase in detection rate would result in increased surveillance and intervention, increased cost, and possibly increased patient anxiety without an increase in diagnoses that persist to term .
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 emptying the 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 well-visualized both 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 placental 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 visualizing 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 .
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 [58-60]. 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 . 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".)
TVUS 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 [62-65].
●Thick (>10 mm) placental edge and/or angle between the basal and chorionic plates greater than 45 degrees [66,67].
●Echo-free space in the placental edge over the internal os .
●Cervical length ≤30 mm [67,69,70].
●Second to third trimester decrease in cervical length by >6 mm or third-trimester cervical length ≤35 mm [71,72].
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 higher cost and more limited availability of MRI . MRI is most useful for diagnosis of complicated placenta previa, such as previa-accreta spectrum . (See "Placenta accreta spectrum: Clinical features, diagnosis, and potential consequences", section on 'Magnetic resonance imaging (MRI)'.)
Placenta previa-accreta spectrum — When placenta previa is diagnosed, the possibility of placenta 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 patients with placenta previa undergoing cesarean birth, the frequency of PAS at cesarean increased with an increasing number of cesareans as follows :
●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
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 [58,75-77].
●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 [9,43,78-83], but not all [45,46,78,84-86], and remains controversial. If a reduction in fetal growth occurs, it is likely to be mild or related to confounding factors. (See "Fetal growth restriction: Evaluation".)
●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 [9,46,87].
MORBIDITY AND MORTALITY
Maternal — Maternal morbidity from placenta previa is primarily related to antepartum and/or postpartum hemorrhage . In systematic reviews, 52 percent of patients with placenta previa had antepartum bleeding (95% CI 42.7-60.6 percent)  and 22 percent had postpartum hemorrhage (95% CI 15.8-28.7 percent) . Because of the propensity for bleeding, patients with placenta previa are more likely to receive blood transfusions and undergo postpartum hysterectomy. In a study of primary cesarean births for placenta previa (without placenta accreta spectrum [PAS]), the risk of red blood cell transfusion was increased nearly fourfold (aRR 3.8, 95% CI 2.5-5.7) and the risk of hysterectomy was increased over fivefold (aRR 5.1, 95% CI 1.5-17.3) compared with no previa at cesarean . These risks are particularly high for patients with both previa and PAS. In a large series, composite maternal morbidity in patients with placenta previa and zero, one, two, or three prior cesarean births was 15, 23, 59, and 83 percent, respectively, and almost all of the excess composite maternal morbidity was related to complications associated with PAS, such as peripartum hysterectomy . (See "Placenta accreta spectrum: Clinical features, diagnosis, and potential consequences", section on 'Consequences'.)
In resource-abundant countries, the maternal mortality rate associated with placenta previa is less than 1 percent  but remains high in resource-poor countries where maternal anemia, lack of medical resources, and home births are more common . Severe maternal morbidity and maternal mortality in patients with placenta previa is usually due to rapid, significant loss of intravascular volume leading to hemodynamic instability, hypoxemia, hypoxia, organ damage, and death. 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 [93-95]. (See "Amniotic fluid embolism".)
Neonatal — The principal causes of neonatal morbidity and mortality are related to preterm birth . In a 2015 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 :
●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)
Others have reported preterm birth <34 weeks in 15 percent of pregnancies, an increased risk for neonatal anemia, and a higher risk of adverse neonatal outcome in patients with recurrent bleeding [50,81,98-100]
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 birth, and improved neonatal care.
Morbidity of low-lying placenta — The morbidity of low-lying placenta (placental edge ≤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 patients with placental edge 1 to 10 mm versus 11 to 20 mm from the internal cervical os reported higher rates of the following :
●Antepartum hemorrhage – 29 versus 3 percent
●Postpartum hemorrhage – 21 versus 10 percent
●Preterm birth – 29 versus 3 percent
●Cesarean birth – 75 versus 31 percent
In a subsequent analysis that evaluated outcomes of patients with persistent low-lying placentas (11 to 20 mm) versus resolved placenta previas (>20 mm), patients attempting a trial of labor in both groups had similar rates of vaginal delivery (77 and 94 percent, respectively) and bleeding outcomes .
RECURRENCE — Placenta previa recurs in 4 to 8 percent of subsequent pregnancies [17,18].
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 patients display sonographic evidence of a placenta previa between 10 and 20 weeks of gestation when they undergo routine obstetric ultrasound examination. The majority 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 patients present with both uterine contractions and bleeding, similar to the presentation of placental abruption. 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 births, and multiple gestation are major risk factors for placenta previa. (See 'Epidemiology' above.)
•Placenta previa should be suspected in any pregnant patient beyond 20 weeks of gestation who presents with vaginal bleeding. For patients 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 patients who have had a previous cesarean birth. (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.)
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