Why pregnancy rates are better with single Day 5 embryo ( blastocyst) transfers

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 ra...
Source: Dr.Malpani's Blog - Category: Reproduction Medicine Source Type: blogs