Vasa previa refers to a situation where there are aberrant fetal vessels crossing over or in close proximity to the internal cervical os, ahead of the fetal presenting part. These vessels are within the amniotic membranes, without the support of the placenta. Vasa previa is a rare but potentially catastrophic cause of antepartum hemorrhage.
Vasa previa can be of two types:
- type I (present in ~ 90% of cases with vasa previa 3): abnormal fetal vessels connect a velamentous cord insertion with the main body of the placenta
- type II
These vessels are unsupported by Wharton jelly or placental tissue and are at risk of rupture during labor.
Vasa previa occurs in 0.6 per 1000 pregnancies 11
Risk factors include:
Prior to rupture of the amniotic membrane during labor, compression of the vulnerable fetal blood vessels by the presenting part may lead to fetal heart decelerations and bradycardia.
However once ruptured, vasa previa will result in brisk vaginal bleeding with rapid fetal exsanguination.
Sonographic features are considered generally specific (~90%) 2.
The diagnosis is often made with transabdominal color Doppler sonography demonstrating flow within vessels which are seen overlying the internal cervical os. Non-Doppler (greyscale) images may suggest the diagnosis if there are echogenic parallel or circular lines within the placenta near the cervix.
Occasionally a transvaginal scan is required to better visualize aberrant vessels. Transvaginal ultrasound has a reported sensitivity of 100% and specificity of 99-99.8% when performed with color Doppler 12.
Treatment and prognosis
If recognized antenatally, vasa previa usually requires an elective cesarean section to avoid the risk of complications during vaginal delivery.
If recognized intrapartum, an emergency cesarean section is usually performed
On antenatal ultrasound consider
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- 3. Hasegawa J, Farina A, Nakamura M et-al. Analysis of the ultrasonographic findings predictive of vasa previa. Prenat. Diagn. 2010;30 (12-13): 1121-5. doi:10.1002/pd.2618 - Pubmed citation
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- 5. Hertzberg BS, Kliewer MA. Vasa previa: prenatal diagnosis by transperineal sonography with Doppler evaluation. J Clin Ultrasound. 1998;26 (8): 405-8. J Clin Ultrasound (link) - Pubmed citation
- 6. Nimmo MJ, Kinsella D, Andrews HS. MRI in pregnancy: the diagnosis of vasa previa by magnetic resonance imaging. Bristol Med Chir J. 1988;103 (2): 12. - Pubmed citation
- 7. Oyelese Y, Jha RC, Moxley MD et-al. Magnetic resonance imaging of vasa praevia. BJOG. 2003;110 (12): 1127-8. BJOG (link) - Pubmed citation
- 8. Gagnon R, Morin L, Bly S et-al. Guidelines for the management of vasa previa. J Obstet Gynaecol Can. 2009;31 (8): 748-60. - Pubmed citation
- 9. Bluth EI. Ultrasound, a practical approach to clinical problems. George Thieme Verlag. (2000) ISBN:0865778612. Read it at Google Books - Find it at Amazon
- 10 .Benson CB, Ultrasonography In Obstetrics And Gynocology, A Practical Approach.. Thieme. (2007) ISBN:1588906124. Read it at Google Books - Find it at Amazon
- 12. L Ruiter, N Kok, J Limpens, JB Derks, IM de Graaf, BWJ Mol, E Pajkrt. Incidence of and risk indicators for vasa praevia: a systematic review. (2016) BJOG: An International Journal of Obstetrics & Gynaecology. 123 (8): 1278. doi:10.1111/1471-0528.13829 - Pubmed
- 12. Ruiter L, Kok N, Limpens J, Derks JB, de Graaf IM, Mol BW, Pajkrt E. Systematic review of accuracy of ultrasound in the diagnosis of vasa previa. (2015) Ultrasound in obstetrics & gynecology : the official journal of the International Society of Ultrasound in Obstetrics and Gynecology. 45 (5): 516-22. doi:10.1002/uog.14752 - Pubmed
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