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Obstetrics » Obstetrics 2nd And 3rd Trimester
Placenta previa
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Presentation Because the patient reported intermittent vaginal bleeding, a sonogram was performed in the second trimester of this pregnancy.
Caption: Sagittal transabdominal US of the LUS.
Description: Sagittal transabdominal sonogram of the lower uterine segment with the bladder distended. The placenta covers the internal os of the cervix.
Caption: Sagittal post void ultrasound of the lower uterine segment.
Description: The bladder has been emptied, but the placenta still completely covers the internal os of the cervix.
Differential Diagnosis Placenta previa
Final Diagnosis Placenta previa
Discussion This case demonstrates an important technical aspect of imaging the placenta - the urinary bladder must be completely empty. With the bladder distended, the anterior lower uterine wall may be compressed, resulting in the false appearance of a placenta previa. However, in this case, the previa persists on the post void image, and is therefore a true, complete placenta previa.

While a complete previa covers the entire internal os, a partial previa covers only a section of the internal os of the cervix. A low lying placenta is defined as a placental margin located 2 cm or less from the internal os of the cervix. This is important to recognize, because a low lying placenta frequently requires a cesarean section because of bleeding at the time of delivery, and therefore must be reported to the obstetrician.

Placenta previa may be detected on a first trimester sonogram. However, the cases identified early often resolve as the pregnancy evolves. In fact, of those detected at 15-19 weeks, only 12% persist to term. However, of those detected at 24-27 weeks, 49% persist, and of those identified at 32-35 weeks, 73% of previas will persist to term. Persistant previa requires a cesarean section delivery.

Risk factors for previa include age over 40 years as well as history of infertility treatment, previous cesarean section or habitual abortions. Previa occurs in 0.4 to 0.5% of pregnancies.

In addition to second trimester bleeding and post partum hemorrhage, pregnancies with a placenta previa have an increased risk of pathologic presentation, placenta abruptio and placenta accreta, congenital malformations and perinatal mortality.
Case References Oppenheimer LW, Farine D. A new classification of placenta previa: measuring progress in obstetrics. Am J Obstet Gynecol. 2009 Sep;201(3):227-9.

Vergani P, Ornaghi S, Pozzi I, Beretta P, Russo FM, Follesa I, Ghidini A. Placenta previa: distance to internal os and mode of delivery. Am J Obstet Gynecol. 2009 Sep;201(3):266.e1-5. Epub 2009 Jul 24.

Hsu TY. Abnormal invasive placentation---placenta previa increta and percreta. Taiwan J Obstet Gynecol. 2009 Mar;48(1):1-2.

Bahar A, Abusham A, Eskandar M, Sobande A, Alsunaidi M. Risk factors and pregnancy outcome in different types of placenta previa. J Obstet Gynaecol Can. 2009 Feb;31(2):126-31.

Bronsteen R, Valice R, Lee W, Blackwell S, Balasubramaniam M, Comstock C. Effect of a low-lying placenta on delivery outcome. Ultrasound Obstet Gynecol. 2009 Feb;33(2):204-8.

Wong HS, Zuccollo J, Tait J, Pringle K. Antenatal topographical assessment of placenta accreta with ultrasound. Aust N Z J Obstet Gynaecol. 2008 Aug;48(4):421-3.

Masselli G, Brunelli R, Casciani E, Polettini E, Piccioni MG, Anceschi M, Gualdi G. Magnetic resonance imaging in the evaluation of placental adhesive disorders: correlation with color Doppler ultrasound. Eur Radiol. 2008 Jun;18(6):1292-9. Epub 2008 Feb 1.

Cho JY, Lee YH, Moon MH, Lee JH. Difference in migration of placenta according to the location and type of placenta previa. J Clin Ultrasound. 2008 Feb;36(2):79-84.

Fuchs I, Dudenhausen JW, Sehouli J, Henrich W. Placenta pathology: disorders  of placental location, placental implantation and cord insertion. Ultraschall Med. 2008 Feb;29(1):4-17; quiz 18-23. English, German.

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