I recall when we were pregnant with one child, our pediatrician told us that the test to detect <a bad thing> had a greater chance of _causing_ some other kind of damage than the chance of our child having the <bad thing> (I forget which). So, please consider that the parent made the best choices they could, and sometimes that means judging the risk of damage versus the chance of having some condition that could have been discovered by a test.
Like other common chromosomal defects, T18 can be detected with a routine noninvasive first trimester ultrasound screening with 95% recall.[0]
Assuming someone received common prenatal care, this would have been detected, at which point termination would have been discussed. If you have the facts, you make the choice of whether or not to have severely disabled child. And make no bones about it, refusing to choose, is a choice.
While we're discussing this, it's important to mention that prenatal screening can only occur late in the first trimester, or more typically early in the second trimester. It's impossible know if physical abnormalities will develop, until they actually start to develop, and that just takes time.
If a screening test indicates a defect, you need to come back for a more detailed diagnostic test. Assuming you can schedule the tests and get the results back quickly, you're hitting up against 20 weeks.[1] 20 weeks -- not coincidentally -- is time that anti-abortion activists target banning abortions.
First trimester ultrasound screening is far from perfect.
For a 30-year old woman, the T21 detection rate is only 84%[1] - and the false positive rate is high at 4%. As age increases, so does the false positive rate - up to a whopping 67% at 49.
So even if you were told the test detected T13/18/21, a decision to terminate the pergancy could mean terminating a healthy fetus. A terrible decision to make.
But there is some better news - there is a relatively new screening test, the so called "Harmony Test", which is a type of non-invasive prenatal test (NIPT) that involves drawing blood from the mother and looking at the fetal DNA that is present[2]. For the most common genetic disorder, T21, the false positive rate is less than 0.1%[3], or 0.3% for all 3 disorders combined[4].
However, at least in the UK, the test is only available privately for around £400.
I didn't make clear in the first paragraph, but ultrasounds are screening tests, not diagnostic tests. The expensive blood tests are diagnostic.
Screening tests are inexpensive, low risk, high recall (i.e. high true positive) tests meant to indicate if high precision (i.e. low false positive, low false negative) test is warranted. Often the diagnostic tests are more invasive, potentially riskier, and more expensive.
You can now test fetal DNA fragments in maternal blood (cffDNA) to detect trisomies with high specificity and sensitivity-- though a positive result is only ~50% confidence, and needs follow-up screening with a riskier method for confirmation. The 0.7% miscarriage risk from follow-up CVS or amniocentesis is more justified in that case.
