Fruit of the Womb
Fruit of the Womb

How to Interpret a "Positive" First Trimester Screen

A reader left the following comment on a post I wrote regarding "Abnormal First Trimester Screening Results." I have gotten this question repeatedly since we have begun offering this procedure for early aneuploidy (chromosomal abnormalities) screening, so I thought both the question and the response might be of general interest to our readers...

"We learned yesterday from first-trimester screening results that we have a 1 in 110 risk of having a baby with Down Syndrome (decreased from my age-related risk at age 41 of 1 in 34). I was told this is still considered a "positive" result, because the standard is 1 in 240. Hearing the counselor call our results "positive" for Down Syndrome was frightening. I appreciate your description of what these results mean. This has helped me to put it in perspective and understand what the risks really are. I'm not sure that labels such as "positive" or "negative" are very helpful in these cases." Anonymous, July 17, 2007

To Anonymous July 17: The results are what they are. You have a 1 in 110 chance of having a baby with Down syndrome. That means you have a 109 out of 110 chance of having a baby that does NOT have Down syndrome. In other words, of 110 women with any combination of the variables (age, blood test results, nuchal translucency measurement, etc.) that produced your 'results' from the database, only 1 of those would be expected to have a baby with Down syndrome (trisomy 21).

Personally, I do not like the "screen positive and negative" way of presenting things to patients. That may suffuce for research purposes but it makes us sound like we're talking out of both sides of our mouths! This is not a black or white test result and it's not fair to present the information to patients in that way! Could you have a baby with Down syndrome? Sure, someone has to be the one; that's true even if you have a 1 in 10,000 result. There are shades of gray that contribute to the interpretation of the test result, and those are the factors you bring to the table. How you use the number to make further decisions regarding additional screening and/or diagnostic testing depends on your personal level of risk tolerance. Do you view the cup as half empty, or half full? Do you bet nickels in Vegas, or $100 dollar bills? How comfortable does the test result make you feel?...and, many other factors that make you who you are.

In the end, if you opt for a diagnostic test, I would suggest an amniocentesis at 16 weeks. The risk of that procedure in experienced hands is generally less than 1 in 1000. The other alternative is to simply consider having a "genetic sonogram" at 18-20 weeks. If the baby is growing well and there are no fetal abnormalities or significant "markers" for aneuploidy (e.g., thickened nuchal skin fold, hypoplastic nasal bone or middle phalanx of the fifth digit, etc.) at that point (again, by an experienced examiner), your a priori risk based on the first trimester screen result (not your age alone) is reduced by at least 60-80% (I usually quote people 90%). Do what is MOST COMFORTABLE for you. Thanks for reading and for your good question.
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About the Author

Dr. Trofatter is an expert on maternal-fetal medicine.