In the workup of vaginal bleeding in pregnancy, ultrasonographic visualization of placentation is critical. A digital examination is contraindicated under these circumstances until placental location is determined secondary to the risk of massive hemorrhage.
Additionally, a thorough abdominal examination to identify uterine tenderness can be useful in differentiating other causative factors for vaginal bleeding, including uterine rupture and placental abruption. Ideally, placental location should be identified during an anatomy scan between 18 and 20 weeks' gestation. In women with either placenta previa or a low-lying placenta, a repeat ultrasonographic evaluation at 32 weeks is indicated for coordination of mode of delivery.
The following laboratory tests are indicated in women with suspected placenta previa:
Rh compatibility test
levels of fibrin split products (FSP) and fibrinogen
Prothrombin time (PT)/activated partial thromboplastin time (aPTT)
Blood type and cross; hold for at least 4 units
Complete blood cell (CBC) count
Amniocentesis and fetal lung maturity testing, if necessary
Other tests that may be obtained include Kleihauer-Betke test, if there is concern about fetal-maternal transfusion.
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 second- or third-trimester ultrasonography 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.
Transvaginal ultrasonography is considered the gold standard for the diagnosis of placenta previa. This imaging modality is accurate, cost-effective, and well tolerated. Several studies have been published indicating the superiority of transvaginal scans (TVS) as compared to the transabdominal (TAS) approach. In an early study, the false-positive and false-negative rates of TVS were 1.0 % and 2.0%, respectively, with rates of 7% and 8%, respectively, for TAS. [16, 17]
The angle between the transvaginal probe and the cervical canal is such that the probe does not enter the cervical canal. Some clinicians advocate insertion of the probe no more than 3 cm for visualization of the placenta to avoid inadvertent insertion into the cervical os.
Transvaginal ultrasonographic techniques can also be used to evaluate the cervical length, when indicated. Shortened cervical lengths have been shown to have an association with need for emergent cesarean delivery at less than 34 weeks' gestation secondary to severe hemorrhage. 
The distance between the placental edge and internal cervical os on transvaginal ultrasonography after 35 weeks’ gestation is also valuable in planning the route of delivery. When the placental edge is greater than 2 cm from the internal cervical os, women can be offered a trial of labor 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.
Transabdominal ultrasonography is a simple, precise, and safe method to visualize the placenta that can often be used in conjunction with TVS when available. This imaging modality can also be used as an alternative to TVS; however, it is less accurate, with the false-positive and false-negative rates reported as 7% and 8%, respectively.  In one study, 26% of placenta previas diagnosed by transabdominal ultrasonography were found to be misdiagnosed when rescanned using TVS. 
Transperineal/translabial ultrasonography has been suggested as another alternative to transvaginal ultrasonography, especially when instrumentation of the vaginal canal with a probe is a concern. However, it is often deferred to the accuracy, safety and tolerability of transvaginal ultrasonography.
A study suggests that this modality may compliment transabdominal ultrasonography and help to eliminate false-positive results using the transabdominal method alone.
Magnetic Resonance Imaging
Magnetic resonance imaging (MRI) may be used for planning the delivery in that it may help identify placenta accreta (adherence of the placenta to myometrium), placenta increta (invasion through myometrium), or placenta percreta (invasion all the way through the myometrium into serosa, frequently into the bladder). These invasive placental abnormalities are becoming more common (eg, placenta accreta 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 than 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. 
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