As has been the case in many of my recent posts, the topic was chosen based on reader's questions, comments, and concerns left under previous posts. Offering combined first trimester screening for aneuploidy is becoming a 'standard of care' in the U.S. and throughout the world. Patients must understand that this is still a screening test and not a diagnostic test and, as such, is subject to the advantages of the same (i.e., low false negative rates) as well as the disadvantages, primarily, high false positive rates. What this means is that if the risk assessment result returns "negative" there is only a small probability that the baby has Down syndrome or trisomy 18 or 13. However, if the test returns "screen positive" this does not mean the baby has one of those chromsomal abnormalities, but it does justify offering an invasive diagnostic procedure to find out for sure if the patient wants this done.
Smoking has been clearly identified as a factor that can alter the interpretation of the first trimester screen, primarily, by its dose-related lowering effects on pregnancy-associated plasma protein-A (PAPP-A) levels. Fortunately, this was recognized early enough in the accumulation of the reference database that correction for this is included in the interpretation of the test results. Of course, if all women would simply stop smoking (before, during, and after pregnancy) ***HINT,HINT***, we wouldn't have to 'fudge' the data and. perhaps, would have an even more reliable database upon which to provide accurate risk assessment!