This is really a very difficult question because the percentage of embryos from mature eggs that reach the blastocyst stage is very difficult to say. It depends on many factors. It depends on the age of the oocyte, it depends on the quality of the oocyte, depends on the quality of the sperm, of course, and maybe it also depends on the kind of incubators we use to develop the embryos. We always advise our patients to use a time-lapse system in order to have a better selection and maybe a higher number of embryos reaching the blastocyst stage but is really difficult to say what percentage of eggs are going to make it to a blastocyst.
How many embryos reach Day 5? It depends, for example, on the clinical scenario. It is not the same in the younger patient, in older one, it is not the same if we have sperm issues and we can continue naming a lot of reasons but roughly, we can say that at around 20-25% of eggs at the beginning of the treatment, once are fertilised and goes into embryos, at around 20% goes to the stage of blastocyst, so Day 5 embryos.
This is a very interesting topic. However, and prior to answering this question, it should be noted that the number of fertilized oocytes reaching Day 5 blastocyst stage embryos, could widely range between individuals, as in some cases, infertility aetiology could detrimentally affect the embryo’s developmental potential. Under normal circumstances, from the total number of the retrieved oocytes 85% of them are expected to be mature and thus able to be fertilized. From these oocytes it is expected that the 70-80% of them will be normally fertilized giving rise to normal zygotes. Nowadays, both our experience and available equipment and technology are significantly improved, increasing blastocyst formation rate, ranging from 40-60% within normally fertilized zygotes. This means that if we have available 10 normally fertilized oocytes, 4 to 6 of them will successfully reach the blastocyst stage by Day 5 post fertilization.
This again is very individual but we can go through sort of an example of roughly what the stats are. So to work with easy numbers, let’s say that like 12 eggs were retrieved and that’s say it’s a reasonably average number but it can vary from one or two eggs to 20 plus eggs – depending on how a woman’s responded to treatment but say, for example, 12 eggs were retrieved, of the eggs that are collected you can only use mature eggs for IVF treatment and on average about 80% of them will be mature – that gives you 10 eggs all together, so the next day you might have 10 eggs that are available for treatment. Of those 10 eggs, you have about 80% of those will fertilize and become embryos. So you’ve got 12 eggs initially, 10 that are mature and then 8 embryos that have developed. The biggest kind of attrition rate comes from Day 3 to Day 5-6, blastocyst where only about 30 to 50 % of embryos will survive until that point. So that means overall you probably get about maybe three or four, if you’re lucky and good quality blastocyst from 12 eggs that have been collected. So that’s a rough rule of thumb. Women who are a bit younger may be more likely to get more blastocyst based on their egg quality than women who are sort of nearer or past their 40th birthday. It can vary so much from person to person but that’s on average what will happen in an IVF cycle.
This varies so much from patient to patient. For some patients at zero for some patients it’s a hundred and everything in between. There’s too many variables to answer that one.
We expect 80 to 85 % of fertilisation for the eggs but then we expect one on two or even sometimes one on three of the fertilised eggs only getting to the blastocyst stage. There’s a bit of natural selection that is going to happen there. Not every embryo is gonna make it until day 5 but if you have a day 5 embryo it’s a good indication already.
In regard to the standard figures of a normal IVF. In our centre, we know that 80% of eggs that we collect are going to b metaphase II, which means mature. These are the eggs that are suitable for being inseminated with the sperm. From this 80%, again 80% is going to be normally fertilized, so that’s the number of 2 PN or 2 pronuclei, which are the normally fertilized eggs that we expect from the initial pool of mature eggs.
Out of this 80% of fertilized eggs, the number of blastocysts is going to depend a lot on the age of the patient. For example, in the egg donation, we consider it normal to have something between 60 to 70% of the embryos becoming the blastocyst, but if you are working with a patient who is 43 years old, this percentage maybe even zero. There could be an embryo factor.
In standard circumstances, when we think that the egg is not the problem and there is no severe male factor, you should expect that 50-60% of the embryos are going to reach the blastocyst stage.
