probability: pgs

If my 8-cell embryo has 6 normal and 2 abnormal cell. If the embryologist is to pick 2 cells at random, what is the probability that both cells are normal?

 answer: = combin(6,2)/combin(8,2)            (microsoft excel)

              = 15/28

              = 53%

My silly instinct says mosaicism at 8-cell embryo level is not reliable. I will not do PGS. I feel if the embryo passed the PGS screening for genetic defect meaning no trisomy, no down syndrome. Mutation can still happen beyond the 8-cell stage. Somatic mosaicism Down syndrome may also happen.

Knock on woods... If I dont get pregnant, maybe I will try to do the PGS on the frozen embryos.

https://www.khanacademy.org/math/precalculus/prob_comb/prob_combinatorics_precalc/e/probability_with_perm_comb
http://paper.wenweipo.com/2004/05/05/MR0405050001.htm