Purpose or Case Report: Trisomy 18 (Edwards Syndrome) and trisomy 13 (Patau Syndrome) are the second and third most common chromosomal trisomy disorders, respectively, and their characteristic syndromic features have been well described; however, to our knowledge, no prenatal studies have reported the fetal MR findings of trisomy 13 and trisomy 18. For this reason, typical fetal MR features of trisomy 13 and 18 remain incompletely characterized and its role and limitations in its prenatal evaluation and diagnosis are not established in the literature.

In this cased-based, pictorial educational exhibit, we review the syndromic features of trisomy 13 and 18, and present the prenatal MR findings of six cases of trisomy 18 and three cases of trisomy 13, all confirmed by pre and/or postnatal genetic testing. We will discuss the strengths and weaknesses of prenatal MR imaging in the evaluation of trisomy 13 and 18 by comparing to prenatal ultrasound and utilizing postnatal imaging as a reference standard.

After reviewing this presentation, the learner will: (1) Be able to recognize not only the individual fetal MR appearance of abnormalities associated with trisomy 13 and 18, but recognize them as part of a trisomy syndrome. (2) Understand the strengths, limitations, and pitfalls of prenatal MR in evaluation and diagnosis of trisomy 13 and 18. (4) Become familiar with the key points and measurements that should be used in the assessment and diagnosis of the fetus with trisomy 13 and 18 and be included in the radiology report to best assist the multidisciplinary care teams in counseling the pregnant patient and providing care of the fetus and neonate with trisomy 13 and 18.
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Conclusions: Fetal MR imaging is playing an increasing role in the evaluation of known and unknown cases of trisomy 13 and 18 and serves as an important guide for parental counseling, decision-making regarding the pregnancy, delivery planning, and sometimes, controversially, surgical planning. Additionally, in unknown cases of trisomy 13 and 18, where the mother has declined prenatal genetic screening or amniocentesis, fetal MR can assist the obstetrician in finding a unifying diagnosis when a fetus has multiple congenital abnormalities. As such, it is essential that the fetal imaging radiologist be not only able to appropriately recognize and characterize fetal anomalies associated with trisomy 13 and 18 on MRI, but recognize them as part of a trisomy syndrome.