The number is small, but it tells some story. That's why I don't do 3-day embryo PGS because
1) it hurts the embryo, and
2) it may not be reliable
I prefer to grow blasts instead as a weeding tool, and I understand for our age group, as many as 50% of the blasts can be abnormal, but it is a good bet considering the risk and reward. There are many blast PGS studies and the normal ratio is all over the place, but one thing is for sure, many aneuploid embryos simply cannot advance to blast stage (only ~20% abnormal embryos can become blasts).