how to get Z-score without normal samples as control group in nipt(Non-invasive prenatal testing)

Question

[Leon]

Hi, i have a cfDNA sequencing data of a pregnant woman, i want to test the existence of trisomy or not. However, it is not possible to get Z-score value of the sample without a set of normal samples as control group. I think this will be a very useful question for people like me meeting such question. So anyone can give me a alterative method? Thanks!

[Nicolas Rosewick]

Maybe I do not understand your question but a z-score is defined as z = (x - x̄ )/ s where x̄ is the sample mean and s the standard deviation. Could you maybe use cfDNA of the other chromosomes as control ? You should describe a little bit more your problem.