Here is how we arrive at the “over 90%” figure:

Only 1/8 of the mothers started on dex will actually be carrying a child in the target population, i.e., a fetus who is female (46,XX) with 21-Hydroxylase Deficiency CAH. That is because half of the fetuses (4/8) will be male, and only 1/4 of the fetuses will have CAH. So only 1 fetus in 8 will be a female with CAH. That (1/8) comes to 12.5%.

Of the 12.5% who are in the target population, researchers estimate that only 80% will have genital virilization prevented by the dex treatment. (Some researchers say the actual number who benefit is lower than 80%. Had this treatment been occurring under the rubric of controlled clinical trials, we would have more confidence about the actual rate of benefit. As it is, the data is very weak compared to the number of women who have actually been treated. But we are allowing the “positive” estimate of 80% in this calculation, to give proponents of dex the benefit of the doubt.) Eighty percent of 12.5% is 10%.

Thus, only 10% will benefit, meaning 90% of fetuses started on the dex treatment in the first trimester of pregnancy will obtain no benefit from the treatment. They will be exposed to the risks.

The actual figure is probably higher than 90% because of paternity confusion in a few cases. We therefore feel safe saying that “over 90% of fetuses will obtain no benefit from the treatment.”

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