Embryo drop off is immense and a v stark reminder of how staggeringly inefficient humans are as a species when it comes to reproduction
The quantity of eggs produced depends on the protocol used and the woman's ovarian reserve. Both quantity and quality decline with age
Not all eggs retrieved will be mature - only mature eggs are capable of being fertilised.
There's no way to tell which eggs are good quality (& worth fertilising) and which are poor & unlikely to be able to make a baby. If you have a larger haul than expected and for whatever reason don't want to risk ending up with surplus embryos you can opt to fertilise some eggs & freeze (or discard) the others, but it's pot luck whether you select decent ones or not
Not all mature eggs will fertilise - this is primarily determined by egg quality. Fertilisation rate varies but approx 75% is considered decent. Unless you have sperm issues, doing ICSI (where the embryologist injects a single sperm cell directly into the egg) doesn't increase fertilisation rates vs conventional IVF (put eggs and sperm in a dish together and they self inseminate)
Not all 2PN embryos will make it to cleavage stage (day 3)
The biggest drop off is typically between day 3 and day 5, when the embryo develops rapidly from cleavage stage (4-6 cells) to blastocyst, a much more complex embryo where the cells have started to differentiate - the inner cell mass (the bit that hopefully becomes fetus) and the trophectoderm (the outer bit that hopefully becomes placenta)
Drop off can vary - if ~50% of day 3 embryos make it to blast that's considered decent
Of those embryos that make it to blastocyst the % that have the potential to become a person is primarily driven by age
This image shows how egg quality declines with age meaning fertility declines and likelihood of miscarriage increases
images.app.goo.gl/BYvQpNbNby9gyqj8A
Across 3 egg collections I had 55 eggs retrieved, of which 45 were mature, of which 35 fertilised, of which 14 made it to blast - of which genetic screening showed 7 were chromosomally normal (of which I miscarried 2, due to uterine issues, and have 5 left on ice). I was 34 when we made our embryos.
Prof Robert Winston once remarked that humans are almost - but not quite - as inefficient at reproduction as pandas