WithทTAS, there is poor visualization of the posterior placenta,
11
the fetal head can interfere with the visualization of the
lower segment,
12
and obesity
13
and underfilling or overfilling of the bladder
14,15
also interfere with accuracy.
For these reasons, TAS is associated with a false positive rate for the
diagnosis of placenta previa of up to 25%.
16
Accuracy rates
for TVS are high (sensitivity 87.5%, specificity 98.8%, positive predictive value 93.3%, negative predictive value
97.6%), establishing TVS as the gold standard for the diagnosis of placenta previa.
17
The only randomized trial to date
comparing TVS and TAS confirmed that TVS is more beneficial.
18
TVS has also been shown to be safe in the presence
of placenta previa,
17,19
even when there is established vaginal bleeding. Magnetic resonance imaging (MRI) will also
accurately image the placenta and is superior to TAS.
20
It is
unlikely that it confers any benefit over TVS for placental
localization, but this has not been properly evaluated.
Furthermore, MRI is not readily available in most units.
WithทTAS, there is poor visualization of the posterior placenta,11the fetal head can interfere with the visualization of thelower segment,12and obesity13and underfilling or overfilling of the bladder14,15also interfere with accuracy. For these reasons, TAS is associated with a false positive rate for thediagnosis of placenta previa of up to 25%.16Accuracy ratesfor TVS are high (sensitivity 87.5%, specificity 98.8%, positive predictive value 93.3%, negative predictive value97.6%), establishing TVS as the gold standard for the diagnosis of placenta previa.17The only randomized trial to datecomparing TVS and TAS confirmed that TVS is more beneficial.18TVS has also been shown to be safe in the presenceof placenta previa,17,19even when there is established vaginal bleeding. Magnetic resonance imaging (MRI) will alsoaccurately image the placenta and is superior to TAS.20It isunlikely that it confers any benefit over TVS for placentallocalization, but this has not been properly evaluated.Furthermore, MRI is not readily available in most units.
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