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Inherited Disorders and Birth Defects

Accurate Down Symdrome Markers

02/07/2007

Question:

I took the blood sampling test at 17 weeks to determine any birth defects. All was normal except the Down syndrome result. My risk showed up as 1 in 25 for having a Down syndrome baby. I am 28 and this is my first baby.

This test was followed by a basic ultrasound. This ultrasound had very normal result except one brain ventricle was enlarged. This lead to a level 2 ultrasound. The level 2 showed a small amount of fluid in both kidneys, enlarged brain ventricles, and an underdeveloped femur and humerus for the amount of weeks I am (currently 20 weeks). I was not given any specific measurements. No other markers were found such as heart problems or a thickening of the neck. I am not interested in an amniocentesis.

Should I have pushed for a second blood test to compare results? How likely are the issues found on the level 2 ultrasound directly related to Down syndrome? Can the found issues be related to something else? I feel like the doctors are pushing for Down syndrome just because one blood test showed an increased risk. Thank you for your time.

Answer:

This question has been forwarded by Urinary and Genital Disorders (Children):

As you know, the screening test you had, a maternal serum screen, looks at different biochemical products that have been found to be associated with chromosome problems; the most common one is Down syndrome, also called trisomy 21. Because this blood test is a screening test, not a diagnostic test - it can only tell you about your potential chance or risk to have a baby with Down syndrome. It cannot tell you for sure if the baby will have Down syndrome.

In your case, if the maternal blood screen calculated your chance to have a baby with Down syndrome was 1 in 25, that means that out of 25 babies, one would have Down syndrome, the others would not.

The level II ultrasound that was done is to look for "markers" that are associated with Down syndrome. This is the appropriate follow-up evaluation to an abnormal maternal serum screen. While markers found on ultrasound are not abnormalities in themselves, they are clues that perhaps the baby has Down syndrome. The markers you list - an enlarged ventricle in the brain, fluid in the kidneys and an underdeveloped femur and humerus - do not have to be associated with Down syndrome, but they are most commonly associated with Down syndrome. Again, these markers can only tell you about your potential risk to have a baby with Down syndrome - they cannot tell you for sure that the baby is affected.

The only way to know for sure if the pregnancy has Down syndrome is to do an amniocentesis. Obtaining fluid from around the baby which contains the baby's cells and doing a chromosome analysis (a diagnostic test) is the only way to tell you whether or not the baby has Down syndrome.

However, there is a chance of complications that can lead to a miscarriage following amniocentesis. Doctors usually say the risk is about a half of a percent (0.5% or 1 in 200), but a recent study suggests that the risk of miscarriage may be less than this, about 0.06% or 1 in 1600 pregnancies. Either way, it is your decision as to whether or not to have the amniocentesis.

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 in which they have a lot of expertise. You can locate a genetics center near you through the National Society of Genetic Counselors Resource Center web site 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