What a normal result honestly means
A normal result lowers some risks by a lot. It does not make a promise about your baby, and it never claimed to.
Here is the whole stage in one paragraph. A negative blood test means the chance of three specific chromosome conditions is now much lower than it was. A normal anatomy scan means about two thirds of the major structural problems that could be seen have been looked for and not found. A negative carrier panel means you and your partner are unlikely to both carry the same one of a particular list of gene changes. Add all three together and you have ruled out a real but narrow set of conditions, against a background in which about 1 in 33 babies is born with a birth defect and four in five of those have no known cause.
ACOG has already written the honest version for patients. On a negative result: A negative result means that your fetus is at lower risk of having the disorder compared with the general population. It does not rule out the possibility that your fetus has the disorder.
On a positive one: A positive screening test result for aneuploidy means that your fetus is at higher risk of having the disorder compared with the general population. It does not mean that your fetus definitely has the disorder.
The same ACOG bulletin that lists reassuring patients when results are normal
as a benefit of testing also says, in the paragraph beside it, that testing cannot identify all abnormalities or problems in a fetus
. Both are true. Only the first tends to get said in the room.
The nearest evidence is a survey of 276 pregnant women at a university clinic in Hong Kong in 2008. Of those, 249 (90.2%) said they understood why the scan was being done, and 93 (33.7%) were actually correct about it. That study does not show that women overestimate detection rates; the median detection rate they guessed was close to the real one. What it shows is a gap between feeling informed and being informed.
What I can offer beyond that is a clinical observation from many years of doing this, and I mark it as an observation rather than a measurement: in my experience, most parents leave a normal scan believing considerably more has been ruled out than has been.
"If this result is positive, how likely is it to be correct for someone like me?"
"Is this a screening test or a diagnostic one?"
"After a normal result, what is my remaining chance of a serious problem?"
"Would you have this test yourself, and why?"