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Immediately obtain conventional cytogenetics for any child or neonate with suspected Patau syndrome, unless cytogenetic diagnosis has been made prenatally. Patau syndrome may occur as a freestanding trisomy of chromosome 13 or, more rarely, as a Robertsonian translocation with an extra copy of chromosome 13 attached to another acrocentric chromosome (eg, 13-15, 21, 22) or as a structural chromosome abnormality wherein only a part of chromosome 13 is duplicated. Perform parental blood chromosome studies in the event that a Robertsonian translocation or other structural chromosome abnormality is found because recurrence risks may be markedly different for freestanding trisomy versus structural rearrangements.
Antecedents to the prenatal diagnosis of Patau syndrome include abnormal first or second trimester aneuploidy screening and abnormal prenatal ultrasonography findings, including birth defects or growth restriction. One or more of these often precedes cytogenetic analysis using chorionic villous sampling (10-13 weeks' gestation) or amniocentesis (15 weeks' gestation or later) in an effort to obtain a small piece of placental tissue or amniocytes from within amniotic fluid, respectively. Newer techniques of noninvasive prenatal testing using cfDNA from maternal blood sample are very promising for having high sensitivity and specificity to detect aneuploidy prenatally. Positive predictive values also appear to be much higher than for standard prenatal aneuploidy screening. 
- In addition to conventional cytogenetics, fluorescent in-situ hybridization (FISH) on interphase cells could be used to obtain a more rapid diagnosis. Cytogenetic analysis is a necessary step in the prenatal diagnosis of Patau syndrome.
- Caveats concerning the limitations of aneuploidy screening in cases of Patau syndrome are warranted. Second trimester screening, which uses maternal serum alpha fetoprotein (MSAFP), human chorionic gonadotropin (hCG), unconjugated estriol, and inhibin, is often insensitive. A significant number of cases can be detected through the use of first trimester screening (ie, ultrasonography measurement of the nuchal translucency, pregnancy associated plasma protein A [PAPPA], and some form of hCG [usually the free circulating form]).
- Birth defects identified in fetuses with trisomy 13 include but are not limited to abnormalities of the CNS, cardiac defects, facial defects, growth restriction, holoprosencephaly, and renal abnormalities.
Mosaicism for trisomy 13 is associated with a milder degree of severity, with the mildest expression typically in the lowest levels of mosaicism. Higher levels of mosaicism are more closely associated with full constitutional trisomy. Because of the possibility of different levels of mosaicism in different tissues, no level of mosaicism can be presumed benign.
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Initiate appropriate imaging studies when holoprosencephaly or cardiac or renal anomalies are clinically suspected.
Because of the high frequency of structural defects, perform cardiac evaluations on patients with Patau syndrome who survive the neonatal period.
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