Posted October 30, 2008on:
What is Trisomy 20?
Trisomy 20 is the name given to a chromosome abnormality where cells contain three copies of chromosome 20 (hence tri-somy) rather than the normal two copies. Trisomy 20 can exist in two states. These are:
(i) Mosaic – hence Trisomy 20 Mosaicism
Click below for more information: Trisomy 20 Mosaicism
(ii) Non-Mosaic – Trisomy 20
Non-Mosaic Trisomy 20 (NMT20) is a chromosome abnormality where all cells contain three copies of chromosome 20. NMT20 is not normally compatible with advanced fetal development. The article below describes a pregnancy where NMT20 was detected at amniocentesis and the fetus was examined at autopsy.
Prenatal Diagnosis 2001 May;21(5):387-9
Non-mosaic trisomy 20 presenting at 21 weeks’ gestation as a thoraco-abdominal mass.
Driggers RW, Bernstein H, Lantz M, Stetten G, Escallon CS, Perlman E, Blakemore KJ.
Non-mosaic trisomy 20 is rare in fetuses surviving beyond the first trimester. We report a case of a fetus with non-mosaic trisomy 20 in amniotic fluid cultures obtained during the prenatal evaluation of an unusual thoraco-abdominal mass which was found at autopsy to be pulmonary sequestration. Gross inspection and autopsy of the fetus revealed multiple anomalies.
An older article describes a terminated pregnancy in which NMT20 was detected at amniocentesis, and only minor physiological anomalies were detected in the fetus post-termination.
Clinical Genetics 1989 Apr;35(4):233-6
Non-mosaic trisomy 20 in amniotic fluid cultures with minor anomalies in the fetus.
Myers TL, Prouty LA.
A non-mosaic trisomy 20 was discovered in all cells in two separate cultures from an age-related genetic amniocentesis. Karyotypes of cells obtained via amniocentesis at the time of termination and of cells cultured from the placenta gave the same unambiguous results. However, the fetus, under macro- and microscopic analysis, showed only two minor anomalies: left simian crease and low-set ears. These findings are more suggestive of a normal or at most mosaic trisomy 20 state. The significance of this finding for prenatal diagnosis is discussed.
Amniocentesis and T20 M
Expecting the birth of a new baby deserves to be one of the happiest times in a couple’s life together, and fortunately it often is. Everyone has thoughts about whether the child will be “OK”. For some, though, these thoughts can take over and it can be a time which is affected by worry and doubts. This is often the case where the pregnancy is not straightforward for whatever reason, and not the least when either ultrasound, amniocentesis, CVS or other findings suggest that the child may have a chromosome abnormality. Trisomy 20 Mosaicism (or T20M) is a rare, though not uncommon finding at second trimester amniocentesis or following CVS. As such a finding, however, it can be both frightening and misleading. The latest research has found that in over 90% of cases where T20M has been detected at amniocentesis, no chromosome abnormality is subsequently found in any tissues in the liveborn child. (Wallerstein et al. 2000) Occasionally, the abnormality is, however, confirmed post-natally in extra-embryonic tissues (e.g. cord, amnion, placenta) which, it is believed, may account for many of the findings at amniocentesis.
For a selection of review articles and abstracts on this subject click below: Reviews of Pre-Natal T20M
For a selection of review articles and abstracts of individual cases where T20M was not confirmed post-natally click below: Unconfirmed T20M Cases
For reallife T20M amnio stories click below: T20M Amnio Stories