Hi, there are averages but each person and each round can be so different. A very rough attrition rate could be as follows.
15 eggs retrieved - 13 mature.
10 eggs fertilise.
8 fertilised eggs make it to day three.
4 make it to blastocyst (day 5/6) and can be biopsied.
2 of the blasts are euploid (chromosomally normal) and are therefore suitable for transfer.
I understand that roughly 70% of mature eggs fertilise via ICSI, but there can be a lot of variation round to round. I've done a number of rounds and my fertilisation rate has varied from 84% to 50%. So my overall average fertilisation rate probably is about 70%, but the difference between a 'good' round and a less good round can be quite significant.
The next big one is the drop from day 3 - my clinic says for 35-37 year olds to expect 40-50% of day three embryos to make blastocyst so they can be biopsied.
Of your biopsied blastocysts, my clinic has found that, for 35-37 years olds, 49% of blastocysts are euploid.
The challenge with applying averages to specific situations is that you're working with relatively small numbers, and there's a lot of luck involved. So you could make five blastocysts instead of your four in the example I've given above, or you could make three. It can also feel really random - one round you could get five blastocysts and only one be euploid, and in the next round you could only get two blastocysts, but both of them be euploid.
Hope that helps. Ultimately, each round is a bit of a lottery and you have to try to take deep breaths and not second guess too much. Good luck!