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Inherited Disorders and Birth Defects

Trisomy 18



My triple screen test tested positive for trisomy 18. I already have two healthy children with the same father. Why is this test coming back different? If it is a genetic problem, wouldn`t it have affected the other pregnancies?


The screening test you had, a triple check, looks at 3 types of chemicals that have been found to be associated with chromosome problems; the most common one is Down syndrome, but it can also be associated with trisomy 18.

Trisomy 18 occurs when there are 3 number 18 chromosomes instead of the normal number of 2. Chromosomal abnormalities usually occur at conception when the egg and sperm fuse. If the chromosomes in the egg or sperm do not divide correctly at conception, then the pregnancy may have too many or not enough chromosomal material. In the case of trisomy 18, there is an extra chromosome 18.

While trisomy 18 is a genetic problem, it is not inherited. It is a chance occurrence. So we would expect that you would not have problems with your other pregnancies. Researchers do not know why eggs or sperm may not divide correctly. The one well documented association is maternal age - older mothers (usually beginning about the age of 35) have a greater chance that the chromosomes will not separate properly at conception.

It is important to remember that the triple check is a screening test, not a diagnostic test - it can only tell you about your potential chance or risk to have a baby with trisomy 18. It cannot tell you for sure if the baby will have trisomy 18.

The only way to know for sure if the baby has trisomy 18 is to do a diagnostic test such as a chorionic villus biopsy (CVS) done between 10 and 12 weeks of pregnancy or an amniocentesis done at 15 weeks or later. Both of these procedures collect cells from the tissue or fluid surrounding the developing baby so that the baby’s chromosomes can be examined (a diagnostic test). This is the only way to tell you what your true chance is to have a baby with trisomy 18.

You could also ask your doctor about having a level II ultrasound done to look for “markers” that are associated with trisomy 18. These are findings that are not abnormalities, but are clues that perhaps the baby has trisomy 18. This may give you some more information which would be helpful to you in your decision making process about having a CVS or amniocentesis. Regardless, it is your decision as to whether or not to do this.

I would recommend that you ask your doctor about a referral to talk to a genetic counselor or geneticist to discuss these issues in depth with you. This is an area that they have a lot of expertise. You can locate a genetics center near you through the National Society of Genetic Counselors Resource Center website below.

Related Resources:

National Society of Genetic Counselors Resource Center

For more information:

Go to the Inherited Disorders and Birth Defects health topic, where you can:

Response by:

Anne   Matthews, RN, PhD Anne Matthews, RN, PhD
Associate Professor of Genetics
School of Medicine
Case Western Reserve University