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Comment by robertjpayne | Hacker News Reader

By week 20 there is practically no chance you're not going to know if you're carrying a baby with downs or not unless you refuse all the modern screening/tests available.

NIPT tests can be done at week 8 and give a very high indicator that can be followed up with close monitoring/invasive tests at week 14-15 that give a 99% accuracy. That's hardly "are really not that exact".

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Per Wikipedia, Down's syndrome currently occurs in ~1 in 1000 live births, and used to occur in 2 in 1000 live births some decades ago, in the USA. That means that a test with a 1% false positive rate (99% accuracy) will lead to a false positive for 98-99 healthy embryos per 1000 live births. I would say that this is fair to call "not all that accurate".

Note: I am not in anyway saying that this means that people shouldn't trust the tests, or anything like that. Just reminding everyone that a test's accuracy has to be compared to the incidence of the disease to decide if it's high or not.

> lead to a false positive for 98-99 healthy embryos per 1000 live births.

The number you’re looking for is 9, not 99

Oops... Off by one [order of magnitude]...

Screening ‘test’ vs diagnostic ‘test’ is an important concept.

Screening tests are designed for sensitivity — false positives are expected and identify who would benefit from additional diagnostic tool and procedures.

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