To freeze or not to freeze?... That is the question.....

To freeze or not to freeze?...That is the question....


Many of you are wondering about an article by Zhang et al. published in the New England Journal of Medicine regarding the success rate of transplanting fresh vs frozen embryos.  In recent years, there has been a substantial move towards encouraging frozen embryo transfers rather than fresh embryo transfers.  The concept behind frozen embryo transfers is simple: immediately after a fresh embryo transfer a woman's body is supercharged with estrogen and progesterone, making the uterine environment anything but normal.  In contrast, the relatively low levels of estrogen used for frozen embryo transfers mimics natural conception much more closely.


There are several reasons this is important:


First, the risk of Ovarian Hyperstimulation Syndrome (OHSS) is much greater in fresh embryo transfers compared to frozen embryo cycles, where there is literally no chance of OHSS occurring.  The study did remark that there was a higher risk of OHSS in the fresh transfers.  OHSS can potentially lead to pain, nausea, bloating, severe abdominal swelling, occasionally dangerous blood clots and even hospitalization with the need for drainage of a large build-up of abdominal fluid called ascites.


Second, pregnancies occurring after a frozen embryo transfer have lower risks of complications.  The study did not examine pregnancy complications, only the success rate in achieving pregnancy.  In a recent edition of Fertility and Sterility, a leading fertility scientific publication, Sha et al. reviewed numerous studies comparing fresh vs frozen embryo transfers using a method called meta-analysis to pool study results.  Results demonstrated that frozen embryo transfers resulted in significantly lower risks of placenta previa, placental abruption, low birth weight, very low birth weight, very preterm birth, small for gestational age, and perinatal mortality than fresh transfers.  Some differences that are attributed to the increased risks of pregnancy-induced hypertension, large for gestational age, and postpartum hemorrhage.  


Third, for women over the age of 37, preimplantation genetic screening of embryos is highly recommended to reduce the transfer of genetically abnormal embryos and improve success rates, while reducing the risks of miscarriage.  This cannot be done using fresh embryos in the majority of clinics across North America.



Finally, some patients have asynchronous endometriums.  Say that again, you say?  Yes, your endometrium may not be ready when we are thinking it is.  So we may expect that your endometrium is ready for implantation of a day 5 embryo or day 6 embryo, but it may actually need a little less or a little more time to develop.  This asynchrony is far more common with fresh transfers because the estrogen levels are so much higher than normal, and progesterone levels can rise prematurely causing the endometrium to develop prematurely.  This cannot be achieved without using frozen embryos as this is the only method that allows us to time the progesterone exposure correctly to get your endometrium in sync with your embryo.


Are there any cases that can or should be transferred fresh?

Yes.  In women using donor eggs, there is no reason to freeze the embryos if the endometrium is ready and synchronous.  These women have not been exposed to high levels of estrogen and do not carry any of the associcated risks of an autologous fresh embryo transfer.

Additionally, women trying mini IVF or natural cycle IVF, don't normally develop very high levels of estrogen and progesterone, so they may also undergo fresh transfers, if they do not need endometrial synchronization or preimplantation screening. 


If you have any questions regarding a fresh vs frozen transfer that have not been answered in this blog, please feel free to schedule an appointment with me and we can create a transfer plan that works for you!


Dr. Rahi Victory MD, FRCSC


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