Fruit of the Womb
Fruit of the Womb

What is Aneuploidy Screening?

Patients have a hard time understanding the differences between screening tests and diagnostic tests. The most misunderstandings or misconceptions that seem to arise during pregnancy revolve around screening for aneuploidy (fetal chromosomal abnormalities). Even for providers, this can be a complex and confusing issue that can be difficult and time-consuming to explain to patients and, therefore, often is not given the attention it deserves.

Screening tests are used to provide presumptive identification of an unrecognized or unexpected disease or defect. They are usually designed to be relatively rapid and widely applicable to the population at large. They are not diagnostic, often are associated with what are perceived to be high ‘false positive’ rates, and are meant to have follow-up if ‘abnormal.’ Although, only a small percentage of the ‘abnormal’ tests are usually found to confirm the diagnosis of the abnormality in follow-up, the wide net that is cast by a good screening test does not often miss the opportunity to make the diagnosis either. In the case of aneuploidy, there are several relatively noninvasive approaches for evaluating the fetus ‘at risk’ without putting the fetus at risk by the screening process itself.

Many patients are put off by the screening tests for aneuploidy during pregnancy and the most common reasons cited when they are asked are: “The false positive rates are too high” (which I have found, interestingly enough, often means to them that they think they will be asked to make hard decisions regarding the pregnancy based on the screening test alone and don’t want to take the chance of terminating a normal baby); “I wouldn’t do anything about it anyway”; “I will take what I get and I don’t want to worry about it the whole pregnancy”; and, “My doctor told me that it wasn’t a very good test” (usually related to provider bias or misunderstanding regarding ‘false positive rates’).

Although I wish it wasn’t so, point in fact is that not all pregnancies are perfect; and, these screening tests can help detect completely unexpected aneuploid fetuses in young women who are at ‘low risk’ based on age alone, and also help to guide the ‘at risk’ woman in her decisions regarding the need for invasive diagnostic studies that could, potentially, put the pregnancy at risk by the procedure alone. Indeed, what I have learned in the two years that we have been offering “combined first trimester screening” (the topic for the next post) to ‘at risk’ women, is that most women end up with reassuring results, and sleep better as a consequence, and our number of invasive diagnostic procedures have declined dramatically with no increase in the number of chromosomally abnormal babies missed in women with ‘advanced maternal age.’ I have also learned over the years that some patients know they need to know (and need to be given the opportunity to find out); some patients don’t know they need to know (and benefit from unbiased counseling regarding the value and pitfalls of screening); and many (most?) don’t know what they would do with the information until after they find out.

If you haven’t gotten the message thus far, I believe that ALL pregnant women should be offered aneuploidy screening and I believe more would accept it if they got better counseling. The bottom line is that aneuploidy screening is recommended as a ‘standard of care.’ From the provider’s standpoint, there is proven liability associated with not offering it, not offering it in an unbiased fashion, and with inadequate documentation of informed consent and refusal. The greatest benefit to the patient is that in the vast majority of patients, it can reduce the ‘risk’ and therefore the anxiety related to possibly having an abnormal baby; reduce the need for invasive diagnostic procedures in many cases; and, in situations in which an aneuploid fetus is detected, it can provide time for preparation for the baby, or allow for timely decision-making for those who choose not to continue their pregnancies….
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