Q&A: Are My Amnio Results Accurate?

Posted on Posted in Practical Information, Q&A, Stories, Trisomy 21 (Down syndrome)

The amnio is between 99.4% and 100% accurate.

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QUESTION

We just received our amnio results and were given the unexpected news that our baby girl is positive with Trisomy 21. I have heard so many stories that others were prenatally diagnosed with a Down syndrome baby, and when they went full-term with the pregnancy their baby was fine. I want to be sure there were no mistakes in the lab. Am I just being paranoid?”

ANSWER

I’m really sorry that you’ve received a prenatal diagnosis of Trisomy 21.

Unfortunately, the happy-ending stories we hear about “wrong” prenatal test results are typically based on people not understanding the difference between a screening test and a diagnostic test.

Screening tests like the maternal alpha-fetoprotein test (AFP or MAFP), triple screen, quad screen, nuchal fold translucency, etc. do not diagnose fetal anomalies. They only serve as indicators that chances of a problem existing are statistically higher than normal, and that actual diagnostic tests such as the CVS or amniocentesis and/or high-level ultrasounds performed by maternal-fetal specialist are called for.

When people tell these stories about their co-worker’s cousin’s neighbor’s sister’s friend who got a bad prenatal diagnosis, refused to listen to the doctors, prayed a whole bunch and gave birth to a bouncing miracle baby, the “tests” in question were invariably non-diagnostic screening tests.

Screening tests can have a false-positive rate of five percent or more. In other words, one out 20 mothers getting a screening test could end up with a false positive result. It’s not surprising that a few of them then refuse actual diagnostic testing and go on to have healthy babies. Nor is it surprising that we’re subjected to magical-miracle “the prenatal test was wrong” stories. It’s human nature: people just love dramatic “happy endings” that defy expectations—even when they’re not technically true.

On the other hand, the amnio is a diagnostic test and it is between 99.4% and 100% accurate. Even if you go with the low-end to the 99.4% accuracy rate, most people will still get secondary confirmation of fetal defects from one or more a high-level ultrasounds. There would have to be a lot of reckless malpractice going on for a healthy pregnancy to get misdiagnosed with serious chromosomal or fetal anomalies.

We would never suggest anyone consider ending a wanted pregnancy based on a screening test results alone. By the same token, we don’t suggest ignoring the results of actual diagnostic tests.

I wish I could give you more hope than that, but when one is making a decision this momentous, it is always best to deal in facts.

 

 

 

photo credit: Female Karyotype, 2014 via photopin (license)