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Clinical Reviews

Diagnosis and safe management of placenta previa

As repeat cesareans increase, so do placental abnormalities

October 2006 · Vol. 18, No. 10


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CASE Diagnosis precedes sentinel episode of bleeding

“G.A.” is a 39-year-old gravida 6, para 1041 who was diagnosed with complete placenta previa during a target ultrasound exam performed at 18 weeks for advanced maternal age. She had a sentinel episode of vaginal bleeding at 29 weeks and was hospitalized for close monitoring.

Management strategy

One course of steroid was given, vaginal bleeding subsided, and she was discharged for outpatient conservative management, including iron and folic acid supplementation.

The outcome

The patient progressed to 36 weeks’ gestation, when she underwent amniocentesis to assess fetal lung maturity. When the results were reassuring, a cesarean section was scheduled. Intraoperative blood loss was diminished using pelvic vessel embolization. Surgery was uncomplicated, and a healthy infant was delivered.

Placenta previa is a leading and potentially life-threatening cause of third-trimester bleeding.1 Although the overall incidence is about 0.4% in pregnancies exceeding 20 weeks’ gestation,2 that figure rises with the number of cesarean sections and may reach 10% among women who have undergone 4 or more cesarean deliveries.3 Since more women are requesting elective and repeat cesarean deliveries, we are increasingly likely to encounter this condition.

Fortunately, technological advances have improved maternal and neonatal outcomes after placenta previa:

Nevertheless, the condition necessitates cesarean delivery and can cause serious maternal and perinatal morbidity, including:

It can also occur in association with vasa previa, which, though rare, carries a very high perinatal mortality rate.5

Risk factors

An enlarged placenta or endometrial disruption or scarring in the upper uterine segment due to 1 or more of the factors listed below may increase the likelihood of abnormal placental implantation in the unscarred lower uterine segment:3,6,7

Previa often begins with painless vaginal bleeding

The condition often presents as painless, bright red, vaginal bleeding in the third trimester. It is usually distinguished from abruptio placenta by the absence of abdominal pain and uterine contractions.5 However, approximately 20% of women have uterine activity associated with the first episode of vaginal bleeding.13,14 Moreover, in some cases, painful contractions and labor may precipitate vaginal bleeding from placenta previa.5 Therefore, ultrasound examination is strongly recommended for all women with vaginal bleeding during pregnancy.

Ultrasound for other reasons uncovers many cases

With greater routine use of ultrasonography in obstetrics, a large percentage of women with placenta previa are diagnosed prior to the onset of the characteristic painless vaginal bleeding. In a 2003 study by Dola and colleagues,15 approximately 43% of placenta previa cases were diagnosed by ultrasonography performed for other obstetrical indications prior to the onset of vaginal bleeding.

Look for “warning hemorrhage”

The first episode of vaginal bleeding is rarely profuse or life-threatening to the mother or fetus. The bleeding usually subsides spontaneously, although it could recur and become more severe with subsequent episodes. Pregnancy typically continues after the initial bleeding episode.

The mean gestational age at the time of the first bleeding is 29 to 32 weeks.13,14 However, a third of cases have vaginal bleeding before the 30th week of gestation, a third between 30 and 36 weeks, and a third after 36 weeks’ gestation.13-15 Ten percent of women with the condition may be completely asymptomatic and progress to 38 weeks’ gestation without vaginal bleeding.13,14

Which form of ultrasound is most accurate?

With the advanced technology available today, ultrasound has become the standard means of diagnosing placenta previa.16,17

Transabdominal ultrasound has accuracy as high as 95% and a false-negative rate of 7% in the diagnosis of placenta previa.13,19 However, its accuracy may be adversely affected by maternal obesity, acoustic shadowing of the fetal head in a cephalic presentation, inability to locate the internal cervical os (which is critical for correct diagnosis), and difficulty imaging a posterior placenta and the lateral uterine walls. In addition, a full maternal bladder—usually helpful in transabdominal ultrasound imaging—may cause a false-positive diagnosis if the bladder is overly distended. In this situation, the cervix would appear artificially elongated and give a normally implanted placenta the appearance of encroachment into the internal cervical os.

Transvaginal ultrasound is superior for diagnosis of previa

Leerentveld et al20 reported false-positive and false-negative rates of 1% and 2%, respectively—a striking improvement over transabdominal ultrasound, which has rates of 2% to 6% and 7%, respectively.

Transvaginal sonography has several advantages over transabdominal imaging in localization of the placenta. The shorter distance from the vaginal probe transducer to the cervix and lower uterine segment allows the use of higher-frequency ultrasound waves, with improved resolution; therefore, the relationship between the placental edge and the internal os can be determined more accurately.

Some clinicians may worry that the probe used in transvaginal sonography will disrupt the placenta and provoke significant maternal hemorrhage, but this concern is unfounded. Multiple studies have attested to the safety of transvaginal sonography in localization of the placenta.5,20-22 The probe is introduced and positioned under direct ultrasound guidance at all times, and inadvertent insertion of the endovaginal probe into the internal cervical os is virtually impossible due to the anatomical relationship of the vagina and cervix.21

Transperineal ultrasound is another option. Several investigators have found it to be superior to transabdominal and similarly advantageous to transvaginal sonography in the diagnosis and exclusion of placenta previa.18

Start with transabdominal imaging

In current practice, transabdominal ultrasound is usually performed first to localize the placenta. If there is reason to suspect placenta previa, transvaginal or transperineal sonography is then used to confirm the location of the placenta.

Contractions may hinder imaging

Accurate diagnosis or exclusion of placenta previa may be difficult if uterine contractions are present during ultrasound evaluation. Myometrial contractions shorten the distance between the internal cervical os and the placental edge, altering measurement of this distance. In addition, the ultrasound appearance of a contraction may simulate placental tissue, making it difficult to exclude placenta previa.

We need a new classification system for ultrasound diagnosis

The trouble with tradition

The 4 types of placenta previa in the traditional classification system—complete, partial, marginal, and low-lying—predate the era of ultrasound diagnosis and are based on digital palpation of the placenta through a partly dilated cervical os during labor.

A new system of 3 types

Along with other authors,5,15-18 we propose a new system with 3 categories—complete, incomplete, and low-lying—because ultrasound may not distinguish a placenta partially covering the internal os (a discrete point) from one that is merely encroaching on it.

Complete previa

The placenta completely covers the internal cervical os

Incomplete previa

The placental edge is within 2 cm of the internal cervical os, but does not cover the os

Low-lying previa

The distance from the internal cervical os to the placental edge is between 2 and 3.5 cm

Look for placenta accreta

When placenta previa is diagnosed by ultrasound examination, further diagnostic measures are needed to determine whether placenta accreta is present.5,23 In placenta accreta, neither the normal plane of separation between the placental villi and uterine wall, nor the intervening fibrinoid layer of Nitabuch, is present.5,23

Degrees of abnormal placental implantation

Risk of accreta can reach 67%

There are varying reports on the incidence of placenta accreta, but women with placenta previa and previous cesarean deliveries appear to have the highest incidence.3,23,24 In women with placenta previa and 1 previous cesarean section, the risk of placenta accreta has been estimated at 24%, but it increases to 67% for women with placenta previa and 4 previous cesarean sections.3

Sonographic appearance of placenta accreta

Certain characteristics are suggestive of placenta accreta25,26:

Unfortunately, diagnosis of placenta accreta is difficult prior to delivery, although transvaginal sonography and adjunctive color flow/power Doppler imaging with 2- and 3-dimensional techniques offer improved resolution and have yielded promising results in prenatal diagnosis.27-30

Magnetic resonance imaging (MRI) may also prove useful in detecting placental tissue invasion and evaluating the degree of invasion, especially in a posterior or lateral placenta previa or when there is invasion into the bladder.31-33

Gestational age, symptoms determine management

The management of women with placenta previa in the third trimester depends on the extent of maternal hemorrhage and the fetal gestational age. Clinical categories include:

Some asymptomatic cases resolve

Outpatient management is possible for women who have never bled after diagnosis in the second trimester. These women should abstain from intercourse, avoid digital examination after 20 weeks’ gestation, and immediately present to the hospital if there is any evidence of vaginal bleeding.34

Monthly ultrasound evaluations are necessary to determine whether placenta previa has resolved,34-37 since 90% of cases detected in the second trimester resolve by the third trimester.34 However, if placenta previa persists beyond 24 weeks’ gestation, there is a 50% risk that delivery will be complicated by it.35 If placenta previa persists after 32 weeks, that risk approaches 75%.35

2-fold risk of congenital malformations

Most investigators report a 2-fold increased risk of fetal congenital malformations in cases of placenta previa.5 These malformations include anomalies of the central nervous system, cardiovascular system, respiratory tract, and gastrointestinal tract. Therefore, a target ultrasound examination for fetal anatomy is recommended at the initial ultrasound diagnosis of placenta previa.

Risk of fetal growth restriction warrants heightened surveillance

Some controversy surrounds the incidence of fetal growth restriction in pregnancies complicated by placenta previa. Varma38 reported that fetal growth restriction occurs in 16% of women with placenta previa and is correlated with the number of antepartum bleeding episodes. Other investigators have reported normal fetal growth in women with placenta previa.39 Given this uncertainty, serial follow-up ultrasound evaluations are usually advised for fetal growth assessment.

When patient remains asymptomatic, perform amniocentesis at 36 weeks

Some women progress to the late third trimester without any vaginal bleeding. In these women, amniocentesis is recommended at approximately 36 weeks’ gestation to assess fetal lung maturity.34,40 Elective cesarean delivery can then be planned if pulmonary maturity is documented.

The benefits of elective delivery include a stable patient and an optimally prepared surgical team, as well as the avoidance of emergent surgery, which increases the risk for maternal complications. Emergent surgery also places the fetus at greater risk for anemia, compared with elective procedures(27.7% vs 2.9%, respectively).13

Vaginal bleeding requires inpatient evaluation

Any woman with placenta previa who presents with vaginal bleeding should be admitted to the labor and delivery unit for immediate evaluation of maternal and fetal status, including an estimation of gestational age.

Initial acute care and assessment necessitate34:

If hemorrhage is life-threatening, deliver immediately

During initial evaluation, if the hemorrhage is judged to be massive and life-threatening, resuscitative measures and immediate delivery are necessary to avoid serious maternal morbidity. Recommended measures include constant monitoring of maternal status, aggressive IV fluid resuscitation, transfusion of blood and blood products, assessment of fetal status, and immediate delivery without regard to the maturity of the fetus.

A woman at term or near term (with documented fetal lung maturity) who presents with mild or moderate vaginal bleeding should be delivered via cesarean section.

Conservative management may be appropriate for mild preterm bleeding

If vaginal bleeding is not threatening to the life of the mother, and the fetus is preterm, a conservative approach with aggressive expectant management is appropriate, since most first episodes of vaginal bleeding are self-limited and rarely life-threatening to mother or fetus. Expectant management allows fetal maturation in utero without jeopardizing maternal health. If maternal and fetal health remain stable, the expectant approach allows a safe delay of delivery until the fetus matures.

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