TRANSABDOMINAL ULTRASOUND IN PLACENTA PREVIA

Accuracy 93-97% (1,2).
False - positive rate = 2-6%.
False - negative rate = 2% (3,4).

DIAGNOSTIC PROBLEMS

• Patient obesity increases the distance between the transducer and placenta.
• The edge of the posterior and lateral placenta is often poorly visualized (5).
• The fetal cranium may cast an acoustic shadow over the lower uterine segment.
• An over distended bladder may push the lower uterine segment into a more horizontal position and lead to a false diagnosis of placenta previa (6).

The internal os is only visualized in 70% of cases. A post voiding film minimizes this problem (7).

• Focal contractions of the uterus may result in an image identical to placenta previa (8).

REFERENCES

1. Frieden FJ. Ultrasound in the diagnosis and management of placenta previa at the Mount Sinai Medical Center, 1970-1972 and 1981-1982. Mt Sinai Med J 1987;54:529-534.
2. Langlois SLP, Miller AG. Placenta previa - a review with emphasis on the role of ultrasound. NZ J Obstet Gynecol 1989;29:110-116.
3. Laing FC. Placenta previa: avoiding false negative diagnoses. J Clin Ultrasound 1981;9:109-113.
4. Gorodeski IG, Neri A, Haimovich L, Bahari CM. Placenta previa. The ultrasonographic placental localization and its influence on the mode of delivery. J Reprod Med 1982;27:655-657.
5. Edlestone DI. Placental localization by ultrasound. Clin Obstet Gynecol 1977;20:285-287.
6. Zemlyn S. The effect of the urinary bladder in obstetrical ultrasound. Radiology 1987;128:169-175.
7. Pauzner D, Barrett J, Farine D. Transvaginal scanning in the management of placenta previa. J SOGC 1995;17:231-235.
8. Townsend RR, Laing FC, Nyberg DA et.al. Technical factors responsible for placental migration: sonographic assessment. Radiology 1986;160:105-108.