Is it true you can detect these sorts of thing before birth or is it still a shot in the dark?
To your question, prenatal screening is a thing, but maybe non-invasive techniques (that don’t risk miscarriage) are newer. I don’t know how widely recommended they are throughout the country (US) or world, but they were prescribed for our kiddo born San Francisco this year. They seem to stagger the tests, so that the less invasive ones are used at first, and then the more invasive procedures are recommended if there is information that would indicate an increased risk of whatever condition.
See for example: https://www.ucsfhealth.org/education/prenatal-testing-for-do...
Some might say that these tests are being over used… it’s very personal imo.
The NIPT can screen for the sex of the baby earlier than it can be detected on a scan, so many women elect to have it done.
The first trimester ultrasound is around 96% accurate at detection of down’s syndrome.
This is all to say: indeed they have non-invasive tests and they’re very accessible.
Theirs would be the false positive rate (FPR) or false negative rate (FNR), and yours would be an probability, which takes the FPR (or FNR) rate, the number of tests and the base incidence rate to arrive at 1/300, 1/5000.
For example, in Canada, 1/750 of live born babies has Down's [2].
There are about 400,000 births every year in Canada [3].
So, there are 400,000 / 750 = 500 kids with Down's born every year.
Assume the test has a FNR rate of 0, and a FPR of 5%.
400,000 * 0.05 + 500 = 20,500 children test positive
Odds of positive test indicating Down's (5% FPR): 500/20,500 = 2.4%
Not quite the 1/300, but as you said "soft marker", so maybe the FPR is higher for that, vs all the markers?
To get to 1/300, the FPR rate on the marker would need to be 37.5%?
The 1/5000 is (I believe) a change from a FPR rate to the FNR rate, since the second test is indicating that the disease _isn't_ present.
1/5000 would indicate a negative genetic test will miss one Down's diagnosis in a decade in Canada.
Interestingly, it looks like the FRP increases with age, with it being 4% at age 30 (combined ultrasound/blood test)? [1]
[1] https://pubmed.ncbi.nlm.nih.gov/11702835/
[2] https://www.canada.ca/en/public-health/services/publications...
[3] https://www.statista.com/statistics/443051/number-of-births-...
It's more expensive than the older, inaccurate screening test, so here in the UK it isn't routinely offered to parents, or even explained to them. It's offered by private clinics though.
Also the screen has a sensitivity of 95% with a positive test indicating > 1/300 chance in Down’s syndrome, so the screening is not nearly THAT bad
I was thinking more of false negatives than positives, but reading it back, there was no way for you to know that from what I wrote!
The test for our daughter came back negative (well, something like 3,000,000:1 chance of Down's), but she did actually have Down's. Later, talking with other parents of kids with Down's, multiple had had similar odds, some even crazier (and in a small country with <50k births each year).