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A pregnant patient has a CVS result showing 47,XX,+20[5]/46,XX[20]. Follow-up amniocentesis shows a normal 46,XX karyotype in cultured amniocytes. The pregnancy continues uneventfully and the infant is phenotypically normal at birth.
Mosaic trisomy 20: most often confined placental mosaicism (CPM) detected on CVS, with the trisomic line absent from the fetus on follow-up amniocentesis.
- Most common autosomal trisomy seen as CPM (along with trisomy 7); non-mosaic trisomy 20 is incompatible with life
- True fetal mosaicism (TFM) is rare: most CVS-detected trisomy 20 is confined to extraembryonic tissue
- Origin: usually post-zygotic non-disjunction in the trophoblast lineage; the inner cell mass (fetus) escapes
- Uniparental disomy (UPD) risk: when the trisomy arose by trisomy rescue from a meiosis-I/II error, the rescued disomy may be uniparental. UPD20 is associated with mat-UPD20 syndrome (poor growth, feeding difficulty, hyperinsulinism in some)
- CPM (typical): liveborns are phenotypically normal; pregnancies have a slightly increased risk of intrauterine growth restriction (IUGR) from placental dysfunction from the abnormal trophoblast
- True fetal mosaicism (rare): variable phenotype depending on tissue distribution and percentage:
- Growth restriction
- Spinal anomalies (vertebral malsegmentation, sacral abnormalities)
- Genitourinary anomalies (hypospadias, undescended testes)
- Café-au-lait macules
- Variable cognitive outcome (many normal; some with learning differences)
- Subtle dysmorphic features
- The phenotype does not correlate well with cell-line percentage on amniocentesis because of tissue-specific distribution
- CVS shows mosaic +20: confirm with amniocentesis (samples a different lineage); if amniocentesis is normal, the finding is most likely CPM
- Postnatal: if there were any concerns prenatally, send a skin fibroblast karyotype (not just blood), because mesodermal-derived tissues sometimes harbor the trisomic line when blood is normal, and vice versa
- Serial growth ultrasounds during pregnancy given the IUGR association
- Consider parental UPD studies if the rescued disomy is on chromosome 20
- Prenatal: serial growth ultrasounds given the IUGR association; otherwise expectant management when amniocentesis is normal (likely confined placental mosaicism)
- Postnatal in confirmed true fetal mosaicism: examination and imaging for spinal (vertebral/sacral) and genitourinary anomalies, with referral to the relevant specialists
- Developmental surveillance with early intervention if delays emerge
- Endocrine evaluation for hyperinsulinism/hypoglycemia if maternal UPD20 is identified
- A normal amniocentesis after a +20 CVS is reassuring, but the residual risk for IUGR and (very rarely) a missed TFM is non-zero
- When a CVS-amniocentesis result is discordant, confined placental mosaicism is the leading explanation; trisomies 7, 16, and 22 round out the most commonly involved chromosomes
- Trisomy 16 CPM has the strongest documented IUGR/preeclampsia association; trisomy 20 is milder