Placenta Previa 

Updated: Jan 08, 2018
Author: Ronan Bakker, MD; Chief Editor: Carl V Smith, MD 

Overview

Practice Essentials

Placenta previa is an obstetric complication that classically presents as painless vaginal bleeding in the third trimester secondary to an abnormal placentation near or covering the internal cervical os. However, with the technologic advances in ultrasonography, the diagnosis of placenta previa is commonly made earlier in pregnancy. Historically, there have been three defined types of placenta previa: complete, partial, and marginal. More recently, these definitions have been consolidated into two definitions: complete and marginal previa.

A complete previa is defined as complete coverage of the cervical os by the placenta. If the leading edge of the placenta is less than 2 cm from the internal os, but not fully covering, it is considered a marginal previa (see the following image). Because of the inherent risk of hemorrhage, placenta previa may cause serious morbidity and mortality to both the fetus and the mother.

Placenta previa. Placenta previa.
Complete placenta previa noted on ultrasound. Complete placenta previa noted on ultrasound.
 Another ultrasound image clearly depicting comple Another ultrasound image clearly depicting complete placenta previa.

Pathophysiology

Placental implantation is initiated by the embryo (embryonic plate) adhering in the lower (caudad) uterus. With placental attachment and growth, the developing placenta may cover the cervical os. However, it is thought that a defective decidual vascularization occurs over the cervix, possibly secondary to inflammatory or atrophic changes. As such, sections of the placenta having undergone atrophic changes could persist as a vasa previa.

A leading cause of third-trimester hemorrhage, placenta previa presents classically as painless bleeding. Bleeding is thought to occur in association with the development of the lower uterine segment in the third trimester. Placental attachment is disrupted as this area gradually thins in preparation for the onset of labor; this leads to bleeding at the implantation site, because the uterus is unable to contract adequately and stop the flow of blood from the open vessels. Thrombin release from the bleeding sites promotes uterine contractions and leads to a vicious cycle of bleeding–contractions–placental separation–bleeding.

Etiology

The exact etiology of placenta previa is unknown. The condition may be multifactorial and is postulated to be related to the following risk factors:

  • Advancing maternal age (>35 y)

  • Infertility treatment

  • Multiparity (5% in grand multiparous patients)

  • Multiple gestation

  • Short interpregnancy interval

  • Previous uterine surgery, uterine insult or injury

  • Previous cesarean delivery,[1, 2] including first subsequent pregnancy following a cesarean delivery[1]

  • Previous or recurrent abortions

  • Previous placenta previa (4-8%)

  • Nonwhite ethnicity

  • Low socioeconomic status

  • Smoking

  • Cocaine use

Unlike first-trimester bleeding, second- and third-trimester bleeding is usually due to abnormal placental implantation.

Hemorrhaging, if associated with labor, would be secondary to cervical dilatation and disruption of the placental implantation from the cervix and lower uterine segment. As noted previously, the lower uterine segment is inefficient in contracting and thus cannot constrict vessels as in the uterine corpus, resulting in continued bleeding (see Pathophysiology).

Epidemiology

United States statistics

Placenta previa is frequently reported to occur in 0.5% of all US pregnancies. A large, US population-based, 1989-1997 study indicated an incidence of 2.8 per 1000 live births.[3] The risks increase 1.5- to 5-fold with a history of cesarean delivery. A meta-analysis showed that the rate of placenta previa increases with increasing numbers of cesarean deliveries, with a rate of 1% after 1 cesarean delivery, 2.8% after 3 cesarean deliveries, and as high as 3.7% after 5 cesarean deliveries.[1]

Racial and age-related differences in incidence

The significance of race in having a role in placenta previa is somewhat controversial. Some studies suggest an increased risk among black and Asian women, whereas other studies cite no difference.[4]

Advanced maternal age has also been strongly associated with an increasing incidence of placenta previa. The incidence of placenta previa after age 35 years reported to be 2%. A further increase to 5% is seen after age 40 years, which is a 9-fold increase when compared to females younger than 20 years.[5, 6]

Prognosis

Placenta previa complicates approximately 0.5% of all pregnancies.[4] Technologic advances in ultrasonography have increased the early diagnosis of placenta previa, and several studies have shown that a significant portion of these early diagnoses do not persist until delivery.[7, 8] In fact, 90% of all placentas designated as “low lying” on an early sonogram are no longer present on repeat examination in the third trimester.[9]

However, maternal and fetal complications of placenta previa are well documented. Preterm birth is highly associated with placenta previa, with 16.9% of women delivering at less than 34 weeks and 27.5% delivering between 34 and 37 weeks in a population-based study from 1989 to 1997.[3] There is a significant increase in the risk of postpartum hemorrhage and need for emergency hysterectomy in women with placenta previa.[10]

Maternal complications of placenta previa are summarized as follows:

  • Hemorrhage,[11] including rebleeding (Planning delivery and control of hemorrhage is critical in cases of placenta previa as well as placenta accreta, increta, and percreta.)

  • Higher rates of blood transfusion[11, 12]

  • Placental abruption

  • Preterm delivery

  • Increased incidence of postpartum endometritis[12]

  • Mortality rate (2-3%); in the US, the maternal mortality rate is 0.03%, the great majority of which is related to uterine bleeding and the complication of disseminated intravascular coagulopathy

The Table, below, summarizes the relative risk of some morbidities in women with placenta previa.

Table. Relative Risk of Morbidities in Patients With Placenta Previa (Open Table in a new window)

Morbidities

Relative Risk

Antepartum bleeding

10

Need for hysterectomy

33

Blood transfusion

10

Septicemia

5.5

Thrombophlebitis

5

Endometritis

6.6[12]

 

Complications of placenta previa in the neonate/infant are summarized as follows:

  • Congenital malformations

  • Fetal intrauterine growth retardation (IUGR)

  • Fetal anemia and Rh isoimmunization

  • Abnormal fetal presentation

  • Low birth weight (< 2500 g)[12]

  • Neonatal respiratory distress syndrome[12]

  • Jaundice[12]

  • Admission to the neonatal intensive care unit (NICU)[12]

  • Longer hospital stay[12]

  • Increased risk for infant neurodevelopmental delay and sudden infant death syndrome (SIDS)[13]

  • Neonatal mortality rate: As high as 1.2% in the United States[14]

Patient Education

Patients with placenta previa should decrease activity to avoid rebleeding. In addition, pelvic examinations and intercourse should be avoided.

Counsel patients with placenta previa about the risk of recurrence. Instruct them to notify the obstetrician caring for their next pregnancy about their history of placenta previa.

Encourage patients with known placenta previa to maintain intake of iron and folate as a safety margin in the event of bleeding.

For patient education resources, see Pregnancy Center and Women's Health Center, as well as Bleeding During Pregnancy, and Vaginal Bleeding.

 

Presentation

History

The classic presentation of placenta previa is painless, bright red vaginal bleeding that often stops spontaneously and then recurs with labor.

Vaginal bleeding is most likely to occur in the third trimester. In a study of 179 patients, 33.7% of patients had their first bleeding episode prior to 30 weeks, with 44.6% experiencing bleeding after 30 weeks. Of all the patients with confirmed placenta previa, only 21.7% did not bleed at any time during their pregnancy.[15]

Placenta previa often leads to preterm delivery, with 44% of pregnancies with placenta previa delivered before 37 weeks.[14]

Physical Examination

Any pregnant woman beyond the first trimester who presents with vaginal bleeding requires a speculum examination followed by diagnostic ultrasonography, unless previous documentation confirms no placenta previa.

Because of the risk of provoking life-threatening hemorrhage, a digital examination of the vagina is absolutely contraindicated until placenta previa is excluded.

Findings in a woman with placental previa may include the following:

  • Profuse hemorrhage

  • Hypotension

  • Tachycardia

  • Soft and nontender uterus

  • Normal fetal heart tones (usually)

 

DDx

Diagnostic Considerations

Instruments or fingers should not be placed near the cervix during a vaginal examination, because uncontrolled bleeding can result. Do not perform vaginal or rectal examinations in an outpatient or emergency department setting unless ultrasonography findings have ruled out placenta previa.

Rarely, ultrasonography is unavailable and a digital examination is necessary. If this is the case, the digital examination should be performed in the operating room under double setup conditions (ie, one team ready for emergent cesarean delivery and one team ready for uneventful vaginal delivery).

Other problems to consider in a woman with suspected placenta previa include the following:

  • Vasa previa

  • Cervical or vaginal laceration

  • Vaginal sidewall laceration

  • Miscarriage (spontaneous abortion)

  • Infection

  • Vaginal bleeding

  • Lower genital tract lesions

  • Bloody show

Differential Diagnoses

 

Workup

Approach Considerations

In the workup of vaginal bleeding in pregnancy, ultrasonographic visualization of placentation is critical. A digital examination is contraindicated under these circumstances until placental location is determined secondary to the risk of massive hemorrhage.

Additionally, a thorough abdominal examination to identify uterine tenderness can be useful in differentiating other causative factors for vaginal bleeding, including uterine rupture and placental abruption. Ideally, placental location should be identified during an anatomy scan between 18 and 20 weeks' gestation. In women with either placenta previa or a low-lying placenta, a repeat ultrasonographic evaluation at 32 weeks is indicated for coordination of mode of delivery.

Laboratory Studies

The following laboratory tests are indicated in women with suspected placenta previa:

  • Rh compatibility test

  • levels of fibrin split products (FSP) and fibrinogen

  • Prothrombin time (PT)/activated partial thromboplastin time (aPTT)

  • Blood type and cross; hold for at least 4 units

  • Complete blood cell (CBC) count

  • Amniocentesis and fetal lung maturity testing, if necessary

Other tests that may be obtained include Kleihauer-Betke test, if there is concern about fetal-maternal transfusion.

Ultrasonography

An ultrasonographic evaluation of the fetus is valuable in identifying current gestational age and weight, potential congenital anomalies, malpresentation, and evidence for fetal growth restriction. Ultrasonographic evaluation is also recommended in identifying umbilical cord insertion and excluding a velamentous insertion.

A second- or third-trimester ultrasonography performed for any reason should discover placenta previa if it is present. This is one of the many reasons obstetricians are discouraged from performing limited or target scans in the absence of at least one thorough anatomic assessment.

Transvaginal ultrasonography

Transvaginal ultrasonography is considered the gold standard for the diagnosis of placenta previa. This imaging modality is accurate, cost-effective, and well tolerated. Several studies have been published indicating the superiority of transvaginal scans (TVS) as compared to the transabdominal (TAS) approach. In an early study, the false-positive and false-negative rates of TVS were 1.0 % and 2.0%, respectively, with rates of 7% and 8%, respectively, for TAS.[16, 17]

The angle between the transvaginal probe and the cervical canal is such that the probe does not enter the cervical canal. Some clinicians advocate insertion of the probe no more than 3 cm for visualization of the placenta to avoid inadvertent insertion into the cervical os.

Transvaginal ultrasonographic techniques can also be used to evaluate the cervical length, when indicated. Shortened cervical lengths have been shown to have an association with need for emergent cesarean delivery at less than 34 weeks' gestation secondary to severe hemorrhage.[18]

The distance between the placental edge and internal cervical os on transvaginal ultrasonography after 35 weeks’ gestation is also valuable in planning the route of delivery. When the placental edge is greater than 2 cm from the internal cervical os, women can be offered a trial of labor with a high expectation of success. However, a distance of less than 2 cm from the os is associated with a higher cesarean rate, although vaginal delivery is still possible depending on the clinical circumstances.

Transabdominal ultrasonography

Transabdominal ultrasonography is a simple, precise, and safe method to visualize the placenta that can often be used in conjunction with TVS when available. This imaging modality can also be used as an alternative to TVS; however, it is less accurate, with the false-positive and false-negative rates reported as 7% and 8%, respectively.[16] In one study, 26% of placenta previas diagnosed by transabdominal ultrasonography were found to be misdiagnosed when rescanned using TVS.[19]

Transperineal/translabial ultrasonography

Transperineal/translabial ultrasonography has been suggested as another alternative to transvaginal ultrasonography, especially when instrumentation of the vaginal canal with a probe is a concern. However, it is often deferred to the accuracy, safety and tolerability of transvaginal ultrasonography.

A study suggests that this modality may compliment transabdominal ultrasonography and help to eliminate false-positive results using the transabdominal method alone.

Magnetic Resonance Imaging

Magnetic resonance imaging (MRI) may be used for planning the delivery in that it may help identify placenta accreta (adherence of the placenta to myometrium), placenta increta (invasion through myometrium), or placenta percreta (invasion all the way through the myometrium into serosa, frequently into the bladder). These invasive placental abnormalities are becoming more common (eg, placenta accreta occurs in up to 0.2% of pregnancies) due to the increase in cesarean deliveries,[20] advancing maternal age, hypertensive disease, smoking, and placenta previa cases.

Although in most situations MRI is no more sensitive in diagnosing placenta accreta than ultrasonography, it may be superior for the posterior placenta accreta or the more invasive increta and percreta. For women at high risk for placenta accreta, a 2-step protocol that uses ultrasonography first and then MRI for cases with inconclusive ultrasonographic features may optimize diagnostic accuracy.[21]

A large trial determining the efficacy and safety of the use of MRI during pregnancy has not been performed, and further investigation is required.[22, 23]

 

Treatment

Approach Considerations

Always anticipate massive hemorrhage and preterm delivery in a woman with placenta previa. Document adequate preparation, including transfer to a higher level of care, if necessary.

Hemostasis may be established by one or more of the following:

  • Oversewing the placental implantation site

  • Bilateral uterine artery ligation (O'Leary stitch)

  • Internal iliac artery ligation

  • Circular interrupted ligation around the lower uterine segment both above and below the transverse incision

  • Packing with gauze or tamponade with the Bakri balloon catheter

  • B-lynch stitch

  • Cesarean hysterectomy

Diffuse bleeding often occurs at the implantation site within the lower uterine segment after delivery. [24]  As such, knowledge in the management of acute and heavy blood loss is imperative. Activation of the massive transfusion protocol may be warranted depending on the situation. The use of uterotonics, including methylergonovine maleate (Methergine), 15 methyl prostaglandin F2 alpha (Hemabate), concentrated oxytocin, or misoprostol  are excellent pharmacological agents to help resolve uterine atony, the main cause of hemorrhage post-delivery.

Other options include surgical management, as listed above. Often a combination of medical and surgical interventions are utilized. 

In instances where significant bleeding ensues, rapid replacement of blood products is a priority. In such instances, activation of the Massive Transfusion Protocol is warranted, allowing for stabilization of a patients hemodynamic status by way of rapid supply and infusion of blood products.

A study by Soyama et al that included 266 women with placenta previa evaluated the effectiveness of routine rapid insertion of a Bakri balloon for placenta previa during cesarean section. The study reported smaller bleeding amounts in the Bakri balloon group than in the non-balloon group: intraoperative bleeding (991 vs. 1250 g, p < 0.01), postoperative bleeding (62 vs. 150 g, p < 0.01), and total bleeding (1066 vs. 1451 g, p < 0.01). The study also found that surgical duration was shorter in the balloon group than the non-balloon group (30 vs. 50 min, p < 0.01).<ref>25</ref>

Management of Vaginal Bleeding

Stepwise approach to managing 3rd trimester vaginal bleeding

Patient should be assessed in the labor and delivery unit and the focus should be on maternal hemodynamic stability and fetal well-being. Evaluation should be initiated through close observation of maternal vital signs and initiation of electronic fetal monitoring. Intravenous access is imperative and should be initiated the moment the patient is brought to the labor floor. CBC and T&S should be sent to determine Hg level and possible need for administration of Rh immunoglobulin pending maternal Rh status. If significant vaginal bleeding is noted during evaluation, blood should be cross matched in preparation for rapid replacement of blood volume. Two to four units of blood may be required rather quickly, if hemorrhage ensues. Initiation of massive transfusion protocol may be appropriate if rapid access to blood products is necessary.[13]

Once the patient is deemed stable and fetal well-being has been noted, etiology of the vaginal bleeding can be assessed. Assessment of the placenta should be undertaken through ultrasound by either a transabdominal or transperineal approach. Sterile speculum exam should follow to further assess the quantity and source of the bleeding.  It is imperative that a digital cervical exam is never performed in a patient with concern or ultrasound confirmed placenta previa as this may lead to torrential hemorrhage due to disruption of the placenta and its vessels.[13]

Expectant management with close observation is indicated in situations where the fetal gestational age is less than 36weeks of gestation as long as reassuring fetal monitoring is present and vaginal bleeding has resolved or significantly decreased. Administration of betamethasone should be provided if gestational age is less than 34weeks. If bleeding is severe or non-reassuring fetal monitoring is present then emergency cesarean delivery is indicated.[24]

Indications for hospitalization

For an otherwise uncomplicated pregnancy, continue expectant management in a woman with placenta previa until an episode of bleeding occurs. Studies have not shown any difference regarding maternal or fetal morbidity with home management versus hospitalization prior to the first bleed in these women. Any patient with suspected or known placenta previa and new onset vaginal bleeding should be admitted to the hospital for close monitoring. It is challenging to standardize the duration of a patients hospitalization given the episodic nature of the bleeding. Patients should be closely monitored for a minimum of 48hours during a sentinel bleeding episode. [13] With certain patients, after the initial episode of bleeding has resolved and fetal evaluation has noted to be reassuring, it is appropriate to undergo expectant mgmt. at home. [24] Several studies have shown this to be a safe approach as long as certain criteria are met. [13] However, it is advisable to continue hospitalization in any patient with multiple episodes of bleeding or with those pts with limited access to appropriate medical care if they were to be discharged. [24] At times this may result in hospitalization until delivery.

If bleeding persists and is heavy, preparation for immediate surgery is indicated. In cases where placental location remains uncertain, a double setup examination—in which one management team is prepared for an uneventful vaginal delivery, and a second team is prepared for an immediate cesarean delivery, as needed—may be considered.

Order clotting studies (ie, prothrombin time/activated partial thromboplastin time [PT/aPTT], fibrinogen) if concern arises for disseminated intravascular coagulation (DIC).

Tocolysis

Tocolytics may be considered in cases of minimal bleeding and extreme prematurity in order to administer antenatal corticosteroids. One study appeared to suggest that the use of tocolytics increases the duration of pregnancy and increases the baby's birth weight without causing adverse effects on the mother and the fetus.[26] However, a review article by Bose et al concluded that there is no improvement in perinatal outcome with prolonged tocolytics, and tocolysis beyond 48 hours is not clinically indicated.[14]

If more than one episode of bleeding occurs during gestation (at viability or >24 wk), the clinician should consider hospitalization until delivery, given the increased potential for placental abruption and fetal demise.

Mode of delivery

In general, mode of delivery is directed by the proximity of the leading edge of the placenta in relation to the internal os of the cervix. Several professional organizations and previous studies have recommended elective cesarean delivery when the placenta is les than 2 cm from the internal os. In a retrospective study of 121 pregnancies complicated by placenta previa, 90% of pregnancies with placental edge-to-cervical os distance of 1-2 cm resulted in cesarean deliveries.

Another consideration is the distance to the internal cervical os and the mode of delivery. In general, women with a placental edge–to–cervical os distance that is greater than 2 cm from the internal cervical os can be offered a trial of labor.[27] However, a more recent study by Vergani et al reported that more than two thirds of women with placenta previa who have a placental edge–to–cervical os distance greater than 1 cm deliver vaginally without an increased risk of hemorrhage.[28] The decision to proceed with a trial of labor should be made on an individualized basis at a center that provides 24-hour anesthesia, in-house obstetricians, and trauma blood transfusion protocols.

Surgical Intervention

There is limited data to guide management and as such optimal timing of delivery is controversial. However, in patients with uncomplicated placenta previa, delivery is recommended in the late preterm period between 36weeks 0days to 37weeks 0days of gestation. [29] This provides for the lowest possible risk of bleeding due to labor while also decreasing the risks of prematurity for the fetus.

Earlier delivery may be warranted, however, based on bleeding profile, previous bleeding history, or preterm labor. [24] Each patient and their situation must be taken into individual account.

As noted earlier, the distance between the placental edge and internal cervical os on transvaginal ultrasonography after 35 weeks’ gestation is valuable in planning route of delivery. In general, there is a higher cesarean rate associated with placental edge–to–cervical os distances of less than 2 cm.

In performing cesarean delivery for placenta previa, a low transverse uterine incision is most often used. However, a vertical uterine incision may be considered secondary to an anterior placenta and risk of fetal bleeding.

Invasive placentations

If the patient is at increased risk for invasive placentation (accreta, increta, or percreta), then the patient and surgical team must be prepared prior to delivery. These invasive placentations carry a high mortality rate (7% with placenta accreta) as well as a high morbidity rate (blood transfusion, infection, adjacent organ damage).

Traditionally, uterine atony was the most common cause of cesarean hysterectomy; however, a meta-analysis by Machado showed that abnormal placentation is the most common cause, occurring in up to 45% of cesarean hysterectomies.[30] Risk for cesarean hysterectomy is increased by the presence of complete placenta previa and a history of cesarean delivery or prior abortion.[31]

Controlling blood loss

These complicated pregnancies must have delivery plans that include patient-matched blood and informed consent for possible cesarean hysterectomy. Predelivery placement of balloon catheters for angiographic embolization of pelvic vessels is a technique described in reducing blood loss associated with cesarean hysterectomy and provides the opportunity to manage potential postoperative bleeding with embolization rather than operative re-exploration.[13]

Aortic balloon occlusion prior to cesarean hysterectomy has also been demonstrated to reduce blood loss.[32] Other means to control hemorrhage include the following:

  • B-Lynch or parallel vertical compression sutures

  • Uterine artery ligation (O'Leary stitch)

  • Hypogastric artery ligation

  • Hysterectomy

In the case of a small and focal placenta accreta, resection of the implantation site and primary repair may allow for uterine preservation.

Consultations

Women with placental previa are considered to have high-risk pregnancies, and a multidisciplinary team is involved in their management, including specialists in the following areas:

  • Obstetrics

  • Obstetric anesthesiology

  • Interventional radiology

  • Surgical oncology or general surgery, if extensive surgical dissection is anticipated

  • Gynecologic oncology

  • Urology, if significant involvement of the bladder is anticipated

 

Medication

Medication Summary

No medication is of specific benefit to a patient with placenta previa. Tocolysis may be cautiously considered in some circumstances in order to administer antenatal corticosteroids. A review article concluded that there is no improvement in perinatal outcome with prolonged tocolytics, and tocolysis beyond 48 hours is not clinically indicated.[14]

Encourage patients with known placenta previa to maintain intake of iron and folate as a safety margin in the event of bleeding.

There have been studies that report using prothrombin complex and recombinant factor VII to control hemorrhage associated with obstetric complications and placenta previa.

Tocolytics

Class Summary

Tocolytic agents prevent preterm labor or contractions. Some specialists advocate tocolytics to promote the time for expectant management of symptomatic placenta previa. They should only be used after consultation with an obstetrician.

Magnesium sulfate

Magnesium sulfate is a nutritional supplement in hyperalimentation. It is a cofactor in enzyme systems involved in neurochemical transmission and muscular excitability. In adults, 60-180 mEq of potassium, 10-30 mEq of magnesium, and 10-40 mEq of phosphate per day may be necessary for optimum metabolic response.

Administer magnesium sulfate intravenously (IV) or intramuscularly (IM) for seizure prophylaxis in preeclampsia. Use the IV route for a quicker onset of action in true eclampsia. Discontinue treatment as soon as the desired effect is obtained. Repeat the doses, depending on the continuing presence of a patellar reflex and adequate respiratory function.

Corticosteroids

Class Summary

Steroids may be administered after consultation with a gynecologist, if vaginal bleeding is mild and intermittent, if the patient is not in labor, and if gestation is less than 37 weeks.

Dexamethasone acetate (Baycadron)

Dexamethasone may be given to promote the development of the lungs in the fetus.

Betamethasone (Celestone)

Betamethasone helps to promote fetal lung maturity.

Adrenergic Agonists

Class Summary

These agents have sympathomimetic vasopressor activity.

Terbutaline (Brethine)

Terbutaline acts directly on beta2-receptors to relax uterine contractions.