A second look at genetics

I am just an ignoramus. My imaginations has gone wild because of my desperation to have a genetic counselling. I heard one of a friend's friend twins has Down syndrome. Oh my god, pre implantation genetic screening could have helped prevent its occurrence.

http://downsyndrome.about.com/od/whatcausesdownsyndrome/a/mosaic_ro.htm
*   Understanding the cell division process.
" The zygote starts as one cell, duplicates itself and then divides into two cells. Those two cells each duplicate themselves, divide and create four cells. Those four cells duplicate and divide to create eight cells, and so on. This process of cell duplication and division occurs throughout the life of an individual."

* What is Down Syndrome?
"With full trisomy 21, the presence of an extra number 21 chromosome, the error or misdivision of chromosomes occurs in the egg or sperm cell that becomes the zygote. Because this error was at the very beginning of development, every cell that comes from this zygote will have an extra number 21 chromosome.

With mosaic trisomy 21, the error or misdivision occurs after fertilization at some point during early cell division. Because of this, people with mosaic Down syndrome have two cell lines -- one with the normal number of chromosomes, and one with an extra number 21."

* How Mosiac Down syndrome is detected?
It is detected by blood test on the baby or amniocentesis or cvs during pregnancy.

To summarize, I feel pre-implantation genetic screening is not a reliable screening for mosaic Down syndrome. Pre-implantation genetic screening is usually done at eight-cell stage. It is an invasive test wherein two cells are removed from the embryo. And then screened for chromosomal abnormality.
What if mosaicism exist but the cell picked was good?