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Trisomy 20 ( Mosaic )

What is a chromosome disorder and what is its significance?

Chromosomes are the structures in which our hereditary material (DNA) is packed. A healthy person has 46 chromosomes (23 pairs). One chromosome of each pair (i.e. a total of 23 chromosomes) is donated by each parent to the fetus. Twenty-two pairs are called autosomes, and the 23rd pair consists of the sex chromosomes (X and Y). Females have two X chromosomes and males have one X and one Y. The chromosome pairs are arranged and numbered by size and shape, and each has different genes. Because there are two copies of each chromosome, there is effectively a duplicate gene system (one maternal and the other paternal), each identical in sequence to the other.

A change in the number or structure of the chromosomes in pairs 1- 22, causing an addition or deletion of chromosomal information in these pairs compared with the normal state, constitutes a significant medical problem. In the great majority of cases, this change is expressed as severe structural or functional defects in the tissues and/or mental retardation. For example, an extra number 21 chromosome, resulting in three instead of the normal two so that there are 47 chromosomes altogether, causes Down syndrome. An extra number 18 chromosome, which is larger and therefore contains more genes, manifests as a more severe disorder called trisomy 18. In this syndrome, there are severe defects, and it usually ends in death soon after birth or even in the uterus. This is also the case with a deletion of the whole or a part, however small, of chromosome 18.

Conversely, the clinical manifestation of a change in the number of sex chromosomes (X or Y) or their structure is usually less severe. See under "chromosomes".

Trisomy 20 is an exception

Notwithstanding the above, and in contrast with the situation for other chromosomes, the case of chromosome 20 is different.

First of all, trisomy 20, i.e. the presence of 3 number 20 chromosomes instead of the normal 2, is only found in some of the amniotic and skin cells of the fetus and not in all.

This situation, where there is a mixture of cells with normal chromosomes and cells with trisomy 20, is called mosaicism.

Secondly, trisomy 20 cells never appear in the fetus's blood, so a blood test to look for them should not be carried out.

In some cases, mosaicism of normal cells and cells with trisomy 20 may appear in other fetal tissues, particularly in the urinary tract, and has been demonstrated mainly in cases in which urinary tract defects have been found on ultrasound examination. In a small number of cases defects have been found in other organs.

What is the significance for a fetus that is found to have trisomy 20?

In 10% of cases there is a risk that the fetus will have various defects. These can be detected by ultrasound examination and are found mainly in the urinary tract, but they have been described in other organs as well, such as the heart. Other defects include cleft lip, defects of brain structure including small head circumference, excess cerebrospinal fluid in the ventricles of the brain, etc.; intrauterine growth retardation, etc. Where defects are found, they are usually caused by the presence of the trisomy 20 cells, and in most such cases the pregnancy is terminated.

What about a normal ultrasound picture without intrauterine growth retardation?

The medical literature offers relatively reassuring data about more than 125 cases born out of 152 in which trisomy 20 was detected in the amniotic fluid cells. In about 6.5% of such cases, the newborn was found on examination to be healthy. Some of those who were found to be abnormal had intrauterine growth retardation, so the problem could be anticipated. The percentage of the abnormal cells determines the degree of severity - when there are fewer than 50% abnormal cells the risk that the newborn will have problems is 4.5%, but in this group there are usually warning signs such as intrauterine growth retardation. However, when there are more than 50% abnormal cells, the risk that the newborn will have problems reaches 20%.

Long-term follow-up of development has not been undertaken in a large group of patients with this condition. However, in 18 studies that described patients who were monitored for longer than a month, no neurological problems were observed. Among 7 patients who were monitored for more than a year in whom the percentage of abnormal cells was less than 50%, there were no cases of developmental delay occurring as an isolated finding. There was, however, a single case of delayed development associated with a defect in the structure of the brain that was detected on ultrasound examination. Obviously with such a small number of subjects it is difficult to draw unequivocal conclusions, but at this point in time there is no clear evidence that trisomy 20 has a detrimental effect on development when there are no accompanying defects or intrauterine growth retardation, especially when the percentage of abnormal cells is less than 50%. However, this must be regarded with caution, especially with respect to the possibility of additional problems that have not been demonstrated because of the small numbers. If the pregnancy continues, directed ultrasound examinations in order to monitor the urinary tract, lips and brain must be performed, and growth must also be monitored. Fetal echocardiography should be performed.

When the percentage of abnormal cells is greater than 0%, or ultrasound examination reveals defects or intrauterine growth retardation, there is a relatively increased risk for problems, including developmental problems.
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Comments (19)

Saturday, November 27, 2010 10:14 PM
(1) LR  says:

Trisomy 20

My son was born in 1992 with 69% mosaic trisomy 20. He is a 6'2 handsome athletic, bright, loving individual who is now in college. He's not had any developmental or unusual physical problems. Too bad there are not any significant long-term followup studies of these children to help other parents make decisions on this difficult intra-uterine diagnosis.

Friday, January 14, 2011 2:16 AM
(2) Fernando  says:


LR I want to thank you your citation because we are now in the situation of these difficult intra-uterine diagnosis! Thank you for your personal followup experience...

Wednesday, April 20, 2011 5:29 AM
(3) sue  says:

Trisomy 20 Mosaicism

My son was diagnosed with T20M through CVS (29% of cells abnormal) and then Amnio (25% of cells abnormal). He is 4 1/2 now and completely normal with NO symptoms. It was a VERY stressful pregnancy but we survived it and it was so worth it to have our wonderful, healthy son!

Saturday, April 23, 2011 4:40 PM
(4) Pamela  says:


thank you so much LR and Sue! you have gave me so much hope for our baby, i'm not finding much personal experience online to help. we had a CVS with 50% trisomy 20 the other day, i'm only 12 weeks. If anyone else has some insight for us please email me at pamelah1121@ymail.com thanks so much!!! also if you could direct me to any sites u have found helpful.. ty !

Thursday, July 14, 2011 3:24 PM
(5) nicolette  says:

i have trisomy 20

i was also born in 1992 with trisomy 20 mosaicism. I wish there was more research being done about possible long term effects etc. does anyone know of any research currently being conducted?

Tuesday, July 19, 2011 11:34 PM
(6) Dee  says:


My 11yr old daughter has the syndrome and now she's fighting for her life (were fighting for her life) not everyone will have the same symptoms..She doesnt even remember me anymore, She was fine one day then she wasnt I wish all who have this syndrome, this will never happen to you it affects people in different stages of there lives..God Bless you all

Monday, January 30, 2012 12:55 PM
(7) Diane  says:

my son and mosaic trisomy 20

My son was born on 1992 with mosaic trisomy 20, detected only in the amniotic fluid. I'm embarrassed to say I can't recall the percentage of abnormal cells found. He is a happy sophomore in college and has been healthy his entire life. There wasn't much information for us back then and it looks like there is still not alot known about this condition. We were extrememly worried the whole pregnancy, but no developmental/health issues ever developed.

Wednesday, April 25, 2012 10:28 AM
(8) Jess  says:

Trisomy 20 mosaicism

I am almost 22weeks - cvs found mosaicism confirmed by an amnio with 80% of cells affected. Genetic couselor doesn't know how to advise us and we are beyond upset trying to figure out how to make this decision. I would appreciated all and any honest advice. Please email me at jdest@hotmail.com

Thursday, June 28, 2012 1:18 PM
(9) Karina  says:

My little girl

I am 22 weeks and my little one has been diagnosed with trisomy 20 mosaic. I am not sure how to deal with this. I dont know what to expect or what it really is. I wish I knew more to be able to prepare for my little ones arrival.

Thursday, January 24, 2013 6:25 PM
(10) Kelli  says:

My son was diagnosed with mosaic Trisomy 20

We had a low % of trisomy 20. He is a very healthy happy 3 year old now :) We NEVER. had any issues and he has no symptoms and he is just perfect. It was stressful to hear, but after talking to our geneticist... We knew he would be fine, and he is :)

Friday, December 12, 2014 3:25 PM
(11) Susan  says:

84% but so far so good

My 12month old son was diagnosed with mosaic trisomy 20 on amino (with 84%- huge!) He's super healthy, bang on the 50th percentile for growth and very well coordinated (started walking at 10 months). He does have a mosaic pigmentation pattern on his chest/ back and limbs, but that's not the end of the world. He has a brilliant sense of humour. Cross fingers that the skin thing will be all he gets. I've read that a good percentage of these kids ONLY have the skin thing.

Tuesday, March 03, 2015 9:53 AM
(12) Liz  says:

Miscarried at 8 weeks

I miscarried at 8 weeks, and it was found that the reason was because of trisomy 20. 2 years later, I'm pregnant again with a healthy pregnancy so far.

Tuesday, May 19, 2015 6:44 PM
(13) Tanya  says:

Wish there was more information out there...

I was diagnosed at 17 weeks with 10% Trisomy 20 Mosaicism. I am now 20 weeks and ultrasounds show no abnormalities. My worry is that if nothing is found physically that leaves brain function or developmental problems...or nothing at all! The genetic Drs & counselors only have statistics for less than 50% or more than 50%. My 10% is a LOT less than 50%. I wish there was more research being recorded, so future mothers can be better informed.

Tuesday, September 08, 2015 6:32 AM
(14) Kim  says:

Trisomy 20 and a Perfect Son!

In 2011, I was told I was carrying a little boy with Trisomy 20. My husband and I were worried beyond belief. Jake was born on April 26th, 2011 by C-section, a very healthy little man. He is now 4 1/2, and is the light of our lives. Our little boy is healthy in every way, and is very smart. He is ready to start his second year of preschool. His teachers say he is a bright, loving little man. We encourage parents to believe in these babies, as we are so truly blessed to have our . God bless you.

Friday, September 18, 2015 10:42 AM
(15) Kari Howard  says:

Mosaic Trisomy 20

My niece is 17 years old and, is having major difficulties from this horrendous disorder. She has the rare form of scoliosis and surgery was performed last year--being unsuccessful. She has battled migraines for the past 10 years and, as of today, September 18, 2015, has had a migraine for 26 days. I'm hoping to find someone--anyone that has had the same symptoms, diagnosis, cure for my sweet niece

Monday, November 14, 2016 2:57 AM
(16) Maria  says:

10 week pregnancy

I had a miscarriage after reaching 10 weeks. this was my second miscarriage, we have been doing IVF. I attended the clinic today to get the Cytology results and they told me they were 42XX +20!! Confused.... Ive spent about an hour before finally finding this website to give me answers. I've two more eggs left, not sure whether to use them or give up!!

Friday, December 16, 2016 3:53 AM
(17) Lynn  says:

Trisomy 20

I too have done IVF with pgd and got my results back from my miscarriage (at 8 weeks)and it identifies 47XX + 20. Trisomy 20 was not on the list of abnormalities to test for as it is considered rare. Like you Maria I don't know what to do re my remaining frozen embryos. Have you received any further advice Maria?

Wednesday, December 28, 2016 6:16 AM
(18) jun  says:

detected 20 trisomy mosaic in CVS test

My wife is 19 weeks pregnant We found my baby has 50% of trisomy 20 mosacism in CVS test. So we took an amniotic fluid test 10 days ago, the result will come next week. But the country I live we do not have enough clinical example of trisomy 20 mosaicism, my doctor keeps skeptical about my baby`s condition. But, we see my baby is so healthy and charming in ultrasound test. please leave any comment or information you have about trisomy 20 even if it is good or bad.

Wednesday, December 28, 2016 6:16 AM
(19) jun  says:

detected 20 trisomy mosaic in CVS test

My wife is 19 weeks pregnant We found my baby has 50% of trisomy 20 mosacism in CVS test. So we took an amniotic fluid test 10 days ago, the result will come next week. But the country I live we do not have enough clinical example of trisomy 20 mosaicism, my doctor keeps skeptical about my baby`s condition. But, we see my baby is so healthy and charming in ultrasound test. please leave any comment or information you have about trisomy 20 even if it is good or bad.

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