In order to increase the probability of a pregnancy that takes a large number of eggs are harvested, fertilized, and implanted.
They may have to do the procedure several times, so they have a large number cryogenically stored. If it takes the first time and the parent(s) end up with all the kids they want, you end up with stored frozen embryos.
So do the procedure each time you need embryos. I Still don’t understand the need to have a bunch of extras. Create
those you need not a bunch of spares.
Interesting article here...http://news.yahoo.com/s/nm/20090325/hl_nm/us_reproduction_embryos_3