Placenta Previa

INTRODUCTION

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Background: Placenta previa literally means afterbirth first, and it defines a condition wherein the placenta implants over the cervical os. There can be an implantation completely covering the os (total placenta previa), a placental edge partially covering the os (partial placenta previa), or the placenta approaching the border of the os (marginal placenta previa). A low-lying placenta implants a half to a third of the uterus distinct from the os in the caudad.

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Pathophysiology: Placenta previa is initiated by implantation of the embryo (embryonic plate) in the lower (caudad) uterus. With placental attachment and growth, the cervical os may become covered by the developing placenta. A defective decidual vascularization exists, possibly secondary to inflammatory or atrophic changes.

When an absence of the decidua basalis exists and incomplete development of the fibrinoid layer occurs, the placenta can be attached directly to the myometrium (accreta), invade the myometrium (increta), or penetrate the myometrium (percreta). In general, placenta accreta occurs in approximately 1 of 2500 deliveries. The incidence increases to 10% in women with placenta previa. Maternal age and any uterine surgery (including previous cesarean delivery) increase the risk for placenta accreta. The risk for placenta accreta with placenta previa increases from 4% for those with no surgeries to 65% for those with a history of multiple cesarean deliveries. Two out of 3 patients with placenta accreta require cesarean hysterectomy.

Frequency:
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• In the US: Placenta previa occurs in 0.3-0.5% of all pregnancies; the risk increases by 1.5-5 fold in women with a history of cesarean section. With an increased number of cesarean sections, this risk can be as great as 10%. Other risk factors include advancing maternal age, multiparity (larger placenta), erythroblastosis, history of dilatation and curettage (induced abortion), smoking, and cocaine use.

  Of all types of placenta previa, the frequency of total placenta previa ranges from 20-45%, while partial placenta previa accounts for approximately 30%, and marginal placenta previa accounts for the remaining 25-50%.

Mortality/Morbidity: The perinatal mortality rate associated with placenta previa ranges from 2-3 %.

Race: No racial preponderance in the occurrence of placenta previa exists.

Age: Age is associated with a varying incidence of placenta previa. The risk of placenta previa in relation to age is as follows:

• Aged 12-19 years - 1%
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• Aged 20-29 years - 0.33%

• Aged 30-39 years - 1%

• Older than 40 years - 2%

CLINICAL

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History:

• The classical presentation of placenta previa is painless vaginal bleeding.

• Painless hemorrhage often is noted near the end of the second trimester or in the third trimester.

• Occasionally, this hemorrhage stops spontaneously and then recurs with labor.

Physical:

• Any pregnant patient beyond the first trimester presenting with vaginal bleeding requires a speculum examination followed by ultrasound, unless previous documentation confirms no placenta previa.

• Absolutely no digital examination should be performed until placenta previa is ruled out because this can provoke life-threatening hemorrhage.

• Uterine activity monitoring reveals that approximately 20% of these patients has concurrent contractions with their bleeding.

Causes:

• This bleeding is thought to occur secondary to the thinning of the lower uterine segment in preparation for the onset of labor. The placental attachments become disrupted or tear with this thinning process and cervical dilatation.

• When this bleeding occurs at the implantation site in the lower uterus, the uterus is unable to contract adequately and stop the flow of blood from the open vessels. This is not an issue with placental implantation in the upper uterus secondary to a larger volume of myometrial tissue able to contract and constrict bleeding vessels.

• Other causes of hemorrhage in the setting of placenta previa include digital examination and sexual intercourse.

WORKUP

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Lab Studies:
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• Although coagulopathy is a rare occurrence, a baseline complete blood count with platelets is useful.

• A disseminated intravascular coagulopathy (DIC) profile with prothrombin time (PT), activated partial thromboplastin time (aPTT), fibrinogen, and fibrin split products (FSP) also may be helpful because retroplacental bleeding has been associated with consumptive coagulopathy.

• If the patient’s screening alpha-fetoprotein study was elevated, she may be at increased risk for bleeding and preterm birth.

Imaging Studies:
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• The most useful and least expensive study is sonography.

• While transabdominal sonography has a 95% accuracy, transvaginal sonography provides 100% accuracy in identifying placenta previa.

• However, a phenomenon termed placental migration exists in which placenta previa identified early in pregnancy resolves as the pregnancy proceeds. For example, ultrasounds early in the second trimester identify placenta previa in 5-15% of patients, with 90% of these resolving by term. Similarly, 26% of the total cases of placenta previa and only 2.5% of the cases of partial or marginal placenta previa diagnosed in the second trimester persist into the third trimester. This does not represent true migration of the implantation site but, rather, differential growth of the placenta and distention of the myometrial cavity away from the os.

• MRI is an ideal study for planning the delivery because it allows identification of placenta accreta, placenta increta, and placenta percreta with any placenta previa.

• These rare placental abnormalities carry a very high morbidity and mortality, and may suggest the need for a scheduled cesarean hysterectomy.

• In any case, counsel a patient with placenta previa about the possible eventuality of an emergent cesarean hysterectomy.

Other Tests:
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• Fetal monitoring for reassurance is of primary importance.

• Patients who are Rh negative and unsensitized presenting with painless vaginal bleeding require Rh-immune globulin.

• Also, perform a Kleihauer-Betke test to detect any cases of excessive fetomaternal hemorrhage (>30 mL) that would necessitate additional Rh-immune globulin therapy.

• If placenta previa is identified serendipitously (through an ultrasound ordered for some other reason), continue expectant management until bleeding occurs.

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  • Once bleeding or contractions occur, the patient must rapidly return to the hospital for further evaluation.

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  • While classic teaching required admission from the time of the first bleeding episode until delivery, some studies have shown that no difference in maternal or fetal morbidity exists between home management and hospitalization.

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  • The appropriate candidate for home management must be compliant and have accessible transportation, assistance, and sufficient cognitive function to ensure comprehension of instructions.

• Preterm labor can present as painless vaginal bleeding with placenta previa.

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  • Magnesium sulfate is the tocolytic of choice. A 6-g loading dose followed by 3 g/h or more is required to reduce uterine irritability.

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  • Because of the conflicting information regarding beta-mimetics producing maternal hypotension and tachycardia in the presence of hypovolemia, many clinicians avoid its use.

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  • Exercise care to exclude abruption from the differential diagnosis before tocolysis is undertaken, and continuous fetal monitoring is required during tocolysis.

• If bleeding is minimal and fetal reassurance is noted, consider expectant management to allow for fetal maturity.

• For patients who are preterm (24-36 wk), expectant management is the treatment of choice. Volume replacement, blood transfusion if necessary, and hematocrit maintenance between 30-35% is the goal.

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• However, if the patient is at term (337 wk) with good dating, perform delivery.

• Vaginal delivery may be considered in patients with marginal or partial placenta previa who present in labor with minimal bleeding or in patients with previable gestations or intrauterine fetal demise. In this setting, rigorously perform an evaluation of maternal vital signs and a semiquantitative evaluation of blood loss.

• If bleeding persists, proceed with an immediate cesarean section versus a double setup examination in the operating room.

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• The double setup with full preparation implies having both pediatric and anesthesia personnel in the operating room and having the patient prepped and draped to proceed with emergent cesarean section if necessary. Only consider this if a prompt vaginal delivery is felt to be highly likely.

Surgical Care:

• Cesarean section is the safest mode of delivery for patients with complete placenta previa or significant hemodynamic compromise.

• If time permits, regional anesthesia is the better alternative because general anesthesia is associated with increased blood loss and the need for blood transfusion.

• Most often, the low transverse uterine incision is used; however, a vertical uterine incision may be used when concern about an anterior placenta and fear of fetal bleeding exists.

FOLLOW-UP

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Further Inpatient Care:
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• If hemorrhage occurs monitoring the hemoglobin (Hb) and hematocrit for anemia and transfusing if necessary (eg, Hb <8) is important. Order clotting studies (platelet, PT/INR [international normalized ratio], aPTT, FSP, and fibrinogen) when any concern about DIC exists.

Complications:
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• Hemorrhage is expected in placenta previa secondary to the poor contractility of the lower uterine segment. Therefore, planning for the mode of delivery and obtaining necessary consents are critical steps. Appropriate planning is even more critical in cases of placenta accreta, increta, and percreta.

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

• Oversewing the placental implantation site

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• Localized resection and uterine repair

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• Bilateral uterine artery ligation

• Angiographic embolization of pelvic vessels

• Internal iliac artery ligation

• Curettage of the uterine cavity

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

• Leaving the placenta in situ (needs treatment with antibiotics)

• Packing with gauze

• B-lynch stitch

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• Cesarean hysterectomy

• For patients with known placenta previa, perform the delivery at an institution with both technical and human resources capable of immediate response to massive hemorrhage. Because placenta previa deliveries often are preterm, neonatology resources should also be available.

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• Neonatal complications of placenta previa also include congenital anomalies, respiratory distress syndrome, and anemia. Growth retardation has been implicated but has not been proven to be of greater incidence than the general incidence.

Prognosis:
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• Of placenta previa cases, 50% have preterm delivery, which is a major cause of perinatal mortality.

Patient Education:
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• Counsel patients with placenta previa about the risk of recurrence, and instruct them to notify the obstetrician caring for their next pregnancy regarding their history of placenta previa.

MISCELLANEOUS

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Medical/Legal Pitfalls:
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• The primary medicolegal issue in placenta previa is failure to diagnose. Any second or third trimester ultrasound performed for any reason should identify 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.

• A second pitfall is inadequate preparation and/or counseling. Always anticipate massive hemorrhage and preterm delivery, and document adequate preparation, including transfer to a higher level of care if necessary.