Egg and Embryo Freezing: Vitrification is a Giant Leap
Egg freezing (cryopreservation) of human embryos has been around since the early 1980s but results have not been good. Conventional cryopreservation is a technique still being widely practiced by many IVF clinics. It involves slowly cooling down the embryo until it finally freezes. A major disadvantage of this approach is that it causes ice crystals to form inside the embryo’s cells (blastomeres), damaging them and thus reducing viability.
A few years ago, a breakthrough was reported in the embryo cryopreservation arena. Termed Vitrification, it involves freezing the embryo about 600 times faster than ever before. This ultrarapid process is so fast that it literally allows no time for intracellular ice to form. As a result, vitrification avoids trauma to the embryo. In conventional (slow) freezing, 20-30% of embryos do not survive the freeze-thaw, and those that do survive have less than half the likelihood of generating a pregnancy as do fresh embryos. In contrast, vitrified embryos have a better than 95% freeze-thaw survival rate, and a pregnancy generating potential that is comparable to fresh embryos.
At SIRM, we have been using vitrification for a number of years to cryopreserve both embryos and eggs. When it comes to embryo cryopreservation, we freeze advanced embryos (blastocysts) almost exclusively, since our recent research has shown that embryos failing to advance to the blastocyst (day 5-6) stage are virtually always chromosomally abnormal (aneuploid). Because of this fact, it would be useless to store them or transfer them to the woman’s uterus. By waiting until the blastocyst stage to freeze embryos, we allow non-viable (aneuploid) embryos to be culled out, leaving only those with the best potential to be preserved for future use.
This recent advance in egg/embryo freezing technology came at an opportune time for us at SIRM. We had been working on adapting a genetic testing process called Comparative Genomic Hybridization to check for chromosomal abnormalities in eggs and embryos, but hadn’t found a way to perform the testing in less than a few weeks. In order to use CGH to select chromosomally normal embryos for transfer in an IVF cycle, the turnaround time on the test would have to be less than 5 days. The only other option was to extract the genetic material from the egg/embryo, and freeze the egg/embryo while the test was performed. Prior to the introduction of vitrification, the freeze-thaw survival rate was not high enough to justify the risk of freezing. By combining the two technologies, we could use CGH to identify chromosomally normal embryos for transfer in an IVF cycle and give women a much better chance of having a baby.
Staggered IVF is a form of IVF in which the cycle is split into two segments in order to allow time to genetically test the embryos for chromosomal normality. The 1st segment of the Staggered IVF cycle involves ovarian stimulation, egg retrieval, fertilization, biopsy of the day 3 embryo followed by vitrification of all expanded blastocysts on day 5-6. After the blastocysts have been frozen, the CGH testing is performed on the genetic material.
The 2nd segment of Staggered IVF involves preparing the woman and then transferring one or two CGH-normal blastocysts to her uterus at a time of her choosing (usually a few weeks or months later). Again, the reason for the delay in transferring the embryo(s) is due to the fact that it takes several weeks to perform CGH testing.
While the treating RE might recommend to patients when their embryos should be frozen, it ultimately is the patients’ decision to make. Some RE’s recommend that fertilized eggs (pronucleates or zygotes) be frozen even before cell division takes place. Others prefer to freeze day-3 embryos. The argument given is that by early freezing, there will ultimately be more embryos available for post-thaw processing. While it is a fact that many zygotes and early embryos will not survive to the blastocyst stage, it is also true that those that don’t make it to blastocysts are almost invariably chromosomlly abnormal (aneuploid) anyway and would either fail to attach, or eventually miscarry if they do initially attach.
All available evidence suggests that pregnancies resulting from warmed/thawed embryos are not associated with any increased risk of birth defects.
How We Prepare and Conduct Frozen Embryo Transfer (FET) at SIRM
Within a few days of starting her menstrual period, the recipient begins taking the birth control pill (BCP), 2-4 weeks later she overlaps the BCP with daily injections of a GnRH agonist (Lupron) for 2 days and the BCP is stopped. She continues taking the Lupron until menstruation begins (usually 2-3 days later), whereupon the Lupron dosage is lowered and twice weekly slow-release, intramuscular estradiol valerate (Delestrogen) is administered for about 10 days. The objective is for her blood estradiol (E2) concentration to stabilize at 500pg/ml-1000pg/ml and for her uterine lining to attain a 9mm thickness by ultrasound examination. In cases of inadequate endometrial thickening, vaginal Viagra suppositories are prescribed. This, by improving uterine blood flow will often improve the lining. Intramuscular and/or intravaginal progesterone is administered daily starting about 6 days prior to the FET and continued along with twice weekly intramuscular Delestrogen until until the 10th week of pregnancy or until pregnancy is discounted. Progesterone in oil (PIO) injections are not fun! They hurt and, while remaining the recommended treatment, some women will not tolerate this regimen. In such cases we often recommend vaginal progesterone (e.g. crinone 8%) twice daily.
We recommend oral corticosteroid (dexamethasone or prednisone) supplementation throughout the FET cycle to try and improve immune-receptivity of the uterus. Administration of immunotherapy (with heparin/prednisone and/or Intralipid) is selective and based on a valid medical indication.
The thaw/warm of selected vitrified blastocysts is done several hours prior to the scheduled FET. The aim is to transfer one or two of these at a time.
Blood pregnancy tests are performed 13 days and again 15 days after the first progesterone administration. Contingent upon the detection of properly rising levels of blood hCG levels and subsequently upon the ultrasound confirmation of a viable pregnancy, the administration of daily progesterone and twice weekly Delestrogen are continued until the completion of the 10th week of pregnancy. Needless to say, if pregnancy does not occur, is non-viable or is lost, all medications are appropriately discontinued.
The introduction of Vitrification represents a major “paradigm shift” in the field of human In Vitro Fertilization (IVF). As a result, for the first time we are able to freeze and bank embryos safely and reliably. Vitrification has also vastly improved the potential to successfully bank human eggs. This when combined with the use of CGH on DNA derived from the polar body (PB) of the egg has allowed selective banking of only genetically “competent” eggs, thereby significantly enhancing the birth rate per frozen egg.