Transabdominal ultrasound is a simple, precise, and safe method to visualize the placenta. It has an accuracy of 93% to 98% (Bowie et al., 1978). False-positive results can result from focal uterine contractions or bladder distention. The accurate assessment of placental position can also be difficult with a placenta implanted on the posterior uterine wall. In the patient whose placental edge is not well visualized and is not actively bleeding, the transvaginal approach may be used. The bladder should be empty, and initially the vaginal transducer should be inserted into the vagina to visualize the cervix. Once the closed cervix is visualized, deeper insertion of the probe will aid in visualization of the placenta. The use of the vaginal approach is safe and accurate in diagnosis of a placenta previa.

During routine second-trimester sonography, placenta previa is frequently diagnosed. Typically at 16 weeks' gestation, the placenta occupies 25% to 50% of the uterine surface area. As the third trimester approaches, growth of the lower uterine segment outflanks growth of the placenta, allowing apparent "migration" of the placenta (placen-tal migration) away from the cervical os. For this reason, although 5% of pregnancies are diagnosed with complete previa by second-trimester ultrasound, 90% of these resolve by term (Rizos et al., 1979). When second-trimester sonography suggests a placenta previa, transvaginal ultrasound can be helpful in making a more accurate diagnosis before term.

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