Placenta previa, previa
1. What every clinician should know
Clinical features and incidence
Placenta previa is when placenta overlies or is proximate to the internal os of the cervix.
Complete previa covers the cervical os; partial previa partially covers the cervix but does not completely cross the internal cervical os; low-lying placenta is a placenta in lower uterine segment which is within 2 cm of the internal cervical os.
Placenta previa occurs in 0.3-0.5% of deliveries.
Risk factors for placenta previa include prior cesarean delivery, prior uterine surgery (including D&C), smoking, increasing maternal age, multiparity and multiple gestation.
Relative Risk (RR) of placenta previa is directly proportionate to the number of prior cesarean deliveries (RR estimate approximately 4.5 with 1 prior cesarean delivery and increases to 44.5 with four prior cesarean deliveries).
2. Diagnosis and differential diagnosis
Diagnosis of placenta previa is made by obstetrical ultrasound. Trans-abdominal ultrasound (TAS) is sufficient in many cases, but trans-vaginal ultrasound (TVS) is the gold standard test, particularly when visualization is not optimal trans-abdominally. Confirmation of placenta previa with TVS is often required since the false positive and false negative rates for TAS are 2% and 25% respectively.
The “classical” presentation of placenta previa is development of painless vaginal bleeding during the 2nd or 3rd trimester of pregnancy. This is distinct from “painful” vaginal bleeding, which is characteristic of placental abruption. However, some women with placenta previa may also present with preterm labor and thus have painful contractions.
Many women with placenta previa remain asymptomatic and are diagnosed at routine obstetrical ultrasound evaluation. If placenta previa is diagnosed at 18-20 weeks gestation, approximately 75-80% will resolve before term and the majority resolve by 28-32 weeks.
When a women presents with 2ndor 3rd trimester vaginal bleeding due to placenta previa, differential diagnosis also includes placenta abruption, preterm labor, cervical lesions, or vaginal lacerations.
If a woman with known placenta previa presents with signs of vaginal bleeding, additional work-up should include complete blood count, type and screen, coagulation studies (depending on severity of bleeding), and maternal-fetal medicine consultation.
Depending on location of presentation and gestational age, additional consultation with neonatology or transfer service (to transfer patient to tertiary care unit) may be appropriate.
In women who have placental previa diagnosed at 18-20 weeks, follow-up ultrasound should be performed at 28-32 weeks to assess for resolution.
In asymptomatic women with placenta previa in the 2nd/3rd trimester, some providers prescribe pelvic rest and limited activities to decrease risk of preterm labor or vaginal bleeding (despite the lack of high quality data showing that these are effective strategies).
In women with placenta previa, especially those with prior cesarean deliveries, assessment for evidence of placenta accreta should be undertaken with sonographic examination. These include identification of placenta lakes (especially if “moth eaten” or “swiss cheese appearance”), loss of retroplacental “clear space” or increased vascularity at the placental-uterine interface. Nevertheless, the provider should be aware that even if a placenta accreta is present, signs may not be seen on sonographic (or MRI) assessment.
If a women with placenta previa presents in preterm labor or with vaginal bleeding between 24-34 weeks, efforts should be made to stabilize the mother and prepare for preterm birth. Maternal stabilization includes placement of large bore IVs, rehydration to replace blood loss and evaluation for possible DIC.
Tocolysis may be appropriate depending on maternal status and gestational age, possibly to allow for transfer to tertiary care unit or delay delivery for 24-48 hours. If tocolysis is done, betamimetics and prostaglandin inhibitors should be avoided.
Betamethasone for fetal maturity (betamethasone 12 mg IM q 24 hours x 2) prior to 34 weeks and magnesium sulfate for neuroprotection (6 g load and 2 g/hour) prior to 32 weeks should be administered.
Transfusion of blood products to treat anemia and attempt expectant management is controversial, but may be appropriate if the vaginal bleeding is not excessive and the maternal hemodynamic status remains stable. In these cases, transfusion is usually reserved for hemoglobin less than 8 mg/dL.
In women with uncomplicated complete placenta previa, scheduled delivery between 36 and 37 weeks should be considered.
In women with a placenta previa with additional comorbidities (e.g. high body mass index, multiple previous cesarean deliveries) or complicated clinical course (e.g. multiple episode[s] of vaginal bleeding), earlier delivery may be elected and should be individualized.
In preparation for planned delivery of women with placenta previa, arrangements should be made with blood bank and anesthesia to prepare for possible hemorrhage, prolonged OR time or complicated surgery.
Approximately 16.9% of women with placenta previa deliver before 34 weeks, 27.5% deliver 34-37 weeks, and 55.6% occur after 37 weeks.
In women with placenta previa and a mid-trimester cervical length below 30 mm women delivered before 34 weeks, 34-37 weeks, and after 37 weeks in 45%, 24% and 31% of cases
There is relationship between the risk for complications related to placenta previa and the number of prior cesarean deliveries. The frequency of blood transfusion, presence of placenta accreta, development of DIC, need for hysterotomy and surgical complications all increase as the number of prior cesareans increase (see Table 1).
5. Prognosis and outcome
For women with persistent placenta previa, cesarean delivery is required.
Scheduled delivery at 36-37 weeks is achieved in approximately one-half of women with placenta previa.
Due to placenta location and gestational age at delivery, often the hysterotomy incision must be done in the upper uterine segment.
The risks for hemorrhage, placental accreta, and need for transfusion and hysterectomy all increase with the number of prior cesarean deliveries.
There is an increased chance of placental previa in subsequent pregnancies.
Neonatal morbidity associated with placenta previa is related to gestational age at delivery (see Table 2).
6. What is the evidence for specific management and treatment recommendations
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Dashe, JS. “Toward consistent terminology of placental location”. Semin Perinatol. vol. 37. 2013. pp. 375-9.
Blackwell, SC. “Timing of delivery for women with stable placenta previa”. Semin Perinatol. vol. 35. 2011 Oct. pp. 249-51.
Spong, CY, Mercer, BM, D’alton, M, Kilpatrick, S, Blackwell, S. “Timing of indicated late-preterm and early-term birth”. Obstet Gynecol. vol. 118. 2011. pp. 323-33.
Stafford, IA, Dashe, JS, Shivvers, SA, Alexander, JM. “Ultrasonographic cervical length and risk of hemorrhage in pregnancies with placenta previa”. Obstet Gynecol. vol. 116. 2010 Sep. pp. 595-600.
Grobman, WA, Gersnoviez, R, Landon, MB, Spong, CY, Leveno, KJ. “National Institute of Child Health and Human Development (NICHD) Maternal-Fetal Medicine Units (MFMU) Network. Pregnancy outcomes for women with placenta previa in relation to the number of prior cesarean deliveries”. ObstetGynecol. vol. 110. 2007. pp. 1249-55..
Zlatnik, MG, Cheng, YW, Norton, ME, Thiet, MP, Caughey, AB. “Placenta previa and the risk of preterm delivery”. J Matern Fetal Neonatal Med. vol. 20. 2007. pp. 719-23.
Ananth, CV, Smulian, JC, Vintzileos, AM. “The effect of placenta previa on neonatal mortality: a population-based study in the United States, 1989 through 1997”. Am J Obstet Gynecol. vol. 188. 2003. pp. 1299-304.
Dashe, JS, McIntire, DD, Ramus, RM, Santos-Ramos, R, Twickler, DM. “Persistence of placenta previa according to gestational age at ultrasound detection”. Obstet Gynecol. vol. 99. 2002. pp. 692-7.
Ananth, CV, Demissie, K, Smulian, JC, Vintzileos, AM. “Relationship among placenta previa, fetal growth restriction, and preterm delivery: a population-based study”. Obstet Gynecol. vol. 98. 2001. pp. 299-306.
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- Placenta previa, previa
- 1. What every clinician should know
- 2. Diagnosis and differential diagnosis
- 3. Management
- 4. Complications
- 5. Prognosis and outcome
- 6. What is the evidence for specific management and treatment recommendations