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How freezing all your embryos improves IVF pregnancy rates

Posted Sep 20 2012 9:41pm
Traditionally , we have always transferred fresh embryos in the IVF cycle. Basically, we would select the best quality embryo(s), and transfer this, with the hope that this would implant . If there were spare
( supernumerary ) embryos of good-quality , we would then freeze these; and then transfer them in a later cycle, in case the fresh cycle failed. The transfer of frozen embryos allowed us to maximise the  chances of success.

 Interestingly , lots of clinics are now observing that the pregnancy in frozen cycles seems to be better than with the fresh cycles ! While this may seem counter intuitive, there are multiple reasons for why this is so. For one, our technology for freezing embryos has become much better , because we are using vitrification . As a result of this, 100% of embryos survive after the thaw, if the embryologist is experience and has the required expertise. Even more interestingly , these embryos have a better implantation rate, because the endometrium ( uterine lining) receptivity is much better in frozen cycles as compared to fresh cycles. In a fresh superovulated cycle, our focus is on growing many eggs. The hormones we use for superovulation end up distorting endometrial receptivity , so that even if endometrium appears perfect on the ultrasound scan, it may actually not be optimally receptive to the transferred embryo. On the other hand , when we are transferring frozen embryos , we can use a natural cycle ; or focus on preparing endometrium with hormones, so that it is optimally receptive ; which is why we have a higher success rate when we transfer good-quality embryos into this uterus. After freezing and thawing , the chances of embryo implantation improve considerably, so that many successful clinics now routinely use a “freeze all embryos” policy for their patients.

This approach does have some disadvantages. For one, it takes longer, as patients need to come back for a second cycle, which means patients need to be more patient. Also, it costs more, because of the additional costs of freezing and thawing. Finally, it requires a lot of lab expertise, because you need a full time experienced embryologist to freeze and thaw embryos.
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