Pregnancy rates with blastocyst transfer are potentially "tricky". Because almost all clinics that use extended culture and blastocyst transfer are selective in choosing patients that will be eligible for this procedure, we should expect that pregnancy rates would be higher in this population as compared to the rates seen in "unselected" patients.
This does not mean that having a blastocyst transfer increases a couples chance for pregnancy. It means that (for example) if we select patients for blastocyst transfer that are younger and have more embryos of better quality - we should have higher pregnancy rates - because we already know that those patients are more likely to have success - whether we transfer on day 3 or day 5.
In my opinion, the greatest benefit of blastocyst transfer is to reduce the risk of multiple pregnancy by transferring fewer embryos of higher quality. We have had a very low percentage of triplet (or higher order) pregnancies from blastocyst transfer. Thus far, the rate of triplets has been less than 3% of pregnancies at our center. These triplets have been the result of "identical twin" splitting of an embryo - 2 transferred and 3 implanted.
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