Chromosome analysis: Chromosome analysis is diagnostic for trisomy 13 and can be performed on chorionic villus, amniotic fluid, and peripheral blood.
FISH: Interphase FISH may be performed for rapid enumeration on chorionic villus, amniotic fluid, and peripheral blood.
Non-invasive prenatal testing (NIPT) is available.
TRISOMY 18 (EDWARDS SYNDROME)
Definition
Trisomy 18 is the second most common viable autosomal trisomy. Occurrence is usually sporadic and caused by meiotic nondisjunction; it carries minimal recurrence risk. Risk of trisomy 18 increases with advancing maternal age. This trisomy has a severe phenotype with mental retardation and failure to thrive. Classic clenching of fists may be detected on fetal ultrasound examination. Most trisomy 18 conceptuses abort spontaneously, and about 90% of liveborns die in the 1st year.
Relevant Tests and Diagnostic Value
Maternal serum screen: Risk of trisomy 18 may be calculated with either first- or second-trimester maternal serum screening. Because trisomy 18 is rare, detection rates are not as precise as for Down syndrome, but with a 0.4% false-positive rate, detection rates reportedly range from 60 to 80%.
Chromosome analysis: Chromosome analysis is diagnostic for trisomy 18 and can be performed on chorionic villus, amniotic fluid, and peripheral blood.
FISH: Interphase FISH may be performed for rapid enumeration on chorionic villus, amniotic fluid, and peripheral blood.
Non-invasive prenatal testing (NIPT) is available.
TRISOMY 21 (DOWN SYNDROME)
Definition
Trisomy 21 is the most common viable autosomal trisomy. Individuals with Down syndrome have moderate mental retardation, characteristic dysmorphic features, increased risk of leukemia, and early Alzheimer disease. Cardiac anomalies are common. Risk of trisomy 21 increases with advancing maternal age.
Etiology
Usual causes involve meiotic nondisjunction, resulting in a karyotype of 47,XX(or XY),+21. For these cases, recurrence risk is small, approximately 1% greater than age-related risk for women younger than 35 and no significant risk increase over age-related risk for women older than age 35.
