Clinical Review

Diagnosis and safe management of placenta previa

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As repeat cesareans increase, so do placental abnormalities



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.


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