AUTHOR AND EDITOR INFORMATION
Section 1 of 11  
Author: Saju Joy, MD, MS, Assistant Professor, Division of Maternal-Fetal Medicine, Department of Obstetrics and Gynecology, Wake Forest University School of Medicine
Saju Joy is a member of the following medical societies: American College of Obstetricians and Gynecologists, American Institute of Ultrasound in Medicine, American Medical Association, and Society for Maternal-Fetal Medicine
Coauthor(s):
Deborah Lyon, MD, Director, Division of Benign Gynecology, Associate Professor, Department of Obstetrics and Gynecology, University of Florida Health Science Center at Jacksonville
Editors: Ronald Levine, MD, Director, Section of Gynecologic Endoscopy, Professor, Department of Obstetrics and Gynecology, University of Louisville School of Medicine; Francisco Talavera, PharmD, PhD, Senior Pharmacy Editor, eMedicine; Richard S Legro, MD, Professor, Department of Obstetrics and Gynecology, Division of Reproductive Endocrinology, Pennsylvania State University College of Medicine; Consulting Staff, Milton S Hershey Medical Center; Frederick B Gaupp, MD, Consulting Staff, Department of Family Practice, Assumption Community Hospital; Carl V Smith, MD, The Distinguished Chris J and Marie A Olson Chair of Obstetrics and Gynecology, Professor, Department of Obstetrics and Gynecology, University of Nebraska Medical Center
Author and Editor Disclosure
Synonyms and related keywords:
placenta previa, complete placenta previa, partial placenta previa, marginal placenta previa, low-lying placenta, placenta accreta, placenta increta, placenta percreta, cesarean delivery, cesarean hysterectomy, transvaginal sonography, transvaginal ultrasonography, tocolysis, continuous fetal monitoring, accreta, increta, percreta, transvaginal ultrasound
Background
Placenta previa involves implantation of the placenta over the internal cervical os. Variants include complete implantation over the os (complete 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 in the caudad one half to one third of the uterus or within 2-3 cm from the os.
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. When this occurs, bleeding occurs at the implantation site as 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 a vicious cycle of bleeding-contractions-placental separation-bleeding.
Frequency
United States
Placenta previa occurs in 0.3-0.5% of all pregnancies. The risks increase 1.5- to 5-fold with a history of cesarean delivery. With an increased number of cesarean deliveries, this risk can be as great as 10%. Recent studies show that a previous cesarean delivery did not increase the odds of detecting a placenta previa on second-trimester ultrasonography. However, the rate of placental migration observed at 28-36 weeks' gestation may identify patients who are more likely to deliver vaginally with resolution of the previa. Of all placenta previas, the frequency of complete placenta previa ranges from 20-45%, partial placenta previa accounts for approximately 30%, and marginal placenta previa accounts for the remaining 25-50%.
Mortality/Morbidity
Morbidities
| Relative Risk | | Antepartum bleeding | 10 | | Need for hysterectomy | 33 | | Blood transfusion | 10 | | Septicemia | 5.5 | | Thrombophlebitis | 5 |
The perinatal mortality rate associated with placenta previa ranges from 2-3%.
Maternal mortality is 0.03% in the United States.
Race
Placenta previa has no predilection for any race.
Sex
Placenta previa only occurs in pregnant women.
Age
Age is associated with a varying prevalence of placenta previa. The risk of placenta previa in relation to age is as follows:
- Aged 12-19 years - 1%
- Aged 20-29 years - 0.33%
- Aged 30-39 years - 1%
- Older than 40 years - 2%
History
The classic presentation of placenta previa is painless vaginal bleeding. - Nearly two thirds of symptomatic patients present before 36 weeks' gestation, with half of these patients presenting before 30 weeks' gestation.
- This hemorrhage often stops spontaneously and then recurs with labor.
Physical
- Any pregnant patient beyond the first trimester who presents with vaginal bleeding requires a speculum examination followed by diagnostic ultrasound, unless previous documentation confirms no placenta previa.
- Because of the risk of provoking life-threatening hemorrhage, a digital examination is absolutely contraindicated until placenta previa is excluded.
- Uterine activity monitoring reveals that approximately 20% of patients have concurrent contractions with their bleeding.
Causes
- Hemorrhaging, if associated with labor, would be secondary to cervical dilatation and disruption of the placental implantation from the cervix and lower uterine segment. The lower uterine segment is inefficient in contracting and thus cannot constrict vessels as in the uterine corpus, resulting in continued bleeding.
- Advancing age (>35)
- Multiparity
- Infertility treatment
- Multiple gestation (larger surface area of the placenta)
- Erythroblastosis
- Prior uterine surgery
- Recurrent abortions
- Nonwhite ethnicity
- Low socioeconomic status
- Short interpregnancy interval
- Smoking
- Cocaine use
- Other causes include digital exam, abruption (pre-eclampsia, chronic hypertension, cocaine use, etc) and other causes of trauma (eg, postcoital trauma).
Abruptio Placentae
Cervicitis
Premature Rupture of Membranes
Preterm Labor
Vaginitis
Vulvovaginitis
Other Problems to be Considered
Vasa previa
Cervical or vaginal laceration
Vaginal sidewall laceration
Miscarriage (spontaneous abortion)
Lab Studies
- Although coagulopathy is a rare occurrence, a 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 may also be helpful.
Imaging Studies
- The most useful and inexpensive study is transvaginal ultrasonography that provides 100% accuracy in identifying a placenta previa.
- An alternative would be transabdominal ultrasonography that can be 95% accurate; however, the false-positive and false-negative rates can range from 2-25%. Translabial sonography is another alternative; however, it is often deferred to the accuracy, safety and tolerability of transvaginal sonography.
- MRI may be used for planning the delivery in that it may help identify placenta accreta, placenta increta, or placenta percreta. These invasive placental abnormalities are more common (eg, placenta accrete occurs in up to 0.2% of pregnancies) due to the increase in cesarean deliveries, advancing maternal age, hypertensive disease, smoking, and placenta previa cases. Although in most situations, MRI is no more sensitive in diagnosing placenta accreta that 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.
Other Tests
- 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 sterile speculum examination should be performed to evaluate rupture of the fetal membranes.
Medical Care
For an uncomplicated pregnancy, continue expectant management 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. If, however, bleeding or contractions occur, the patient must rapidly go to the hospital for evaluation and the above mentioned testing. If bleeding persists, and is heavy preparation for immediate surgery is indicated. In cases where placental location remains uncertain, a double setup examination may be considered. However, if bleeding is minimal and fetal reassurance is noted, expectant management may be considered to allow for fetal maturity.
Additionally, tocolytics may also be considered in cases of minimal bleeding and extreme prematurity to administer antenatal corticosteroids. 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.
Surgical Care
The distance between the placental edge and internal cervical os on transvaginal ultrasonography after 35 weeks’ gestation is valuable in planning route of delivery. When the placental edge is greater than 2 cm from the internal cervical os, women can be offered a trial of labour 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.
The timing of delivery is often driven by the patients history and an increased risk for bleeding with advancing gestation. Most authorities recommend delivery at 36-37 weeks' gestation after confirming fetal lung maturity via amniocentesis. However, if the fetal lung maturity testing is immature or is not available, then delivery is often scheduled for 38 weeks' gestation.
Most often a low transverse uterine incision is used; however, a vertical uterine incision may be considered secondary to an anterior placenta and risk of fetal bleeding. 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). 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. Other means to control hemorrhage include B-Lynch or parallel vertical compression sutures, uterine artery ligation, hypogastric artery ligation, and, of course, 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
- Interventional radiology
- Surgical oncology or general surgery if extensive surgical dissection is anticipated
- Gynecologic oncology
- Urology if significant involvement of the bladder is anticipated
No medication is of specific benefit to a patient with placenta previa. Tocolysis may be cautiously considered in some circumstances. Encourage patients with known placenta previa to maintain intake of iron and folate as a safety margin in the event of bleeding.
Drug Category: Tocolytics
Prevent preterm labor or contractions.
| Drug Name | Magnesium sulfate |
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| Description | Nutritional supplement in hyperalimentation; 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 IV or IM for seizure prophylaxis in preeclampsia. Use IV route for quicker onset of action in true eclampsia. Discontinue treatment as soon as desired effect is obtained. Repeat doses dependent on continuing presence of patellar reflex and adequate respiratory function. |
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| Adult Dose | Loading dose: 6 g IV over 20 min; then 2-4 g/h continuous infusion; adjust to lessen contractions; not to exceed 4 g/h |
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| Pediatric Dose | Administer as in adults; alternatively, 20-100 mg/kg/dose IV q4-6h prn; in severe cases, may use doses as high as 200 mg/kg/dose; not to exceed 4 g/h |
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| Contraindications | Documented hypersensitivity; heart block; Addison disease; myocardial damage; myasthenia gravis; impaired renal function; severe hepatitis |
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| Interactions | Concurrent use with nifedipine may cause hypotension and neuromuscular blockade; may increase neuromuscular blockade seen with aminoglycosides and potentiate neuromuscular blockade produced by tubocurarine, vecuronium, and succinylcholine; may increase CNS effects and toxicity of CNS depressants and betamethasone; may increase cardiotoxicity of ritodrine |
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| Pregnancy | A - Safe in pregnancy
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| Precautions | Fetal monitoring is essential, may decrease fetal heart rate; maternal magnesium toxicity may occur at low or high rates of infusion; magnesium may alter cardiac conduction, leading to heart block in patients who are digitalized; monitor respiratory rate, deep tendon reflex, and renal function when electrolytes are administered parenterally; caution when administering magnesium because may produce significant hypertension or asystole; in overdose, calcium gluconate (10-20 mL IV of 10% solution) can be administered as an antidote for clinically significant hypermagnesemia |
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Further Inpatient Care
If hemorrhage occurs, monitor hemoglobin and hematocrit levels for anemia and transfuse if necessary (eg, hemoglobin level <8). Order clotting studies (ie, PT/aPTT, fibrinogen, and fibrin split product) if concern arises for DIC.
Complications
- Hemorrhage (Hemorrhage is expected secondary to the poor contractibility of the lower uterine segment. Planning delivery and control of hemorrhage is critical in cases of placenta previa as well as placenta accreta, increta, and percreta.)
- Preterm delivery
- Congenital malformations
- Abnormal fetal presentation
- Placental abruption
- Hemostasis may be established by one or more of the following:
- Oversewing the placental implantation site
- Bilateral uterine artery ligation
- Internal iliac artery ligation
- Circular interrupted ligation around the lower uterine segment both above and below the transverse incision
- Packing with gauze or tamponading with the Bakri balloon catheter
- B-lynch stitch
- Cesarean hysterectomy
Prognosis
- Fifty percent of women with placenta previa have preterm delivery.
- Those cases complicated with vaginal bleeding and extreme prematurity are at an increased risk of perinatal death.
- A greater incidence of fetal malformations and growth restriction is noted with placenta previa.
Patient Education
Medical/Legal Pitfalls
- The primary medicolegal issue in placenta previa is failure to diagnose. Any second- or third-trimester ultrasonogram 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.
- 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.
| Media file 2:
Placenta previa covering the entire cervical os. |
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| Media file 3:
Placenta previa partially separated from the lower uterine segment. |
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| Media file 4:
Placenta previa invading the lower uterine segment and covering the cervical os. |
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Placenta Previa excerpt Article Last Updated: Jan 2, 2008
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