Logically, one would expect that the pregnancy rate would be better if we transferred two embryos at a time, as compared to putting back only one . Yes, this is true if you calculate the success rate per transfer, but not true when you calculate the cumulative pregnancy rate - the total chances of getting pregnant after transferring all the embryos back.
Let's assume you have 2 top quality blastocysts, and that each blastocyst has has a 30% chance of implanting if we transfer it in an optimally receptive endometrium.
Now if we put two embryos back at the same time, each of them has that 30% chance, and the limiting factor will be endometrial receptivity. If the endometrium is optimal, then there is high possibility that both may implant, which is why the twin pregnancy rate is high after IVF. Sadly, our technology for assessing endometrial receptivity is still crude, and we are forced to depend upon the endometrial thickness and texture as measured on the ultrasound scan to judge this , because the new generation ERA ( Endometrial Receptivity Assay ) tests
are a waste of time and money.
Now the reason why only one implants ( and the other doesn't, even though the uterine lining is receptive) is because one of the blastocysts has a genetic problem ( which we cannot test even with PGS, because PGS only allows us to count the number of chromosomes) , and this prevents it from implanting. This is why transferring two blastocysts has a better pregnancy rate than transferring just one when you calculate the success rate per cycle - you are improving the chances of transferring a genetically normal embryo by putting back two instead of one.
On the other hand, it's possible that when we transfer two genetically normal blastocysts, neither of them will implant because there's a problem with endometrial receptivity. This means no matter how many blastocysts we transfer in that cycle , none of them would implant. If we put two together , we have wasted both these precious embryos . On the other hand , if we put one at a time, each embryo has its own chance of becoming a baby, because the endometrial receptivity may be better in one cycle as compared to another. This way, we are maximizing the probability of achieving a pregnancy for these women , because we are making the best possible use of these blastocysts. I agree this sounds very hit and miss, but sadly these are the current limitations of IVF technology today, and ERA and PGS do not help in overcoming these.
The truth is that each blastocyst is worth its weight in gold , and should be given the best possible chance of becoming a baby. The best way of doing this is by transferring one single blastocyst at a time in an optimally prepared endometrium. Yes, this does have disadvantages , because it does mean that the patient needs to come back again for the next cycle, in case the first one fails.
However, the good thing about transferring frozen embryos after thawing them is that this is a simple procedure , which is not expensive. The patient doesn't need to take any injections and it can be done in a natural cycle as well. Because we can focus on optimizing both the embryo as well as the endometrium , because we're doing these independently, the overall chance of achieving a pregnancy is much better. Sometimes, less is more !
Rather than just calculate pregnancy rate per transfer, patients need to focus on the cumulative conception rate, so that their chances of taking a healthy baby home is maximized.
Is your doctor suggesting you transfer more than one embryo in one cycle ? This suggests he has no confidence in his IVF lab, and you should look for another IVF clinic !
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