Current science and our statistics suggest that frozen embryos may have healthier outcomes than fresh. Therefore, we transfer frozen embryos for several reasons.
At Northern California Fertility Medical Center, embryos may be frozen (cryopreserved) for several reasons. Some of these reasons include:
After an egg retrieval and embryo transfer in an IVF cycle, where there are surplus embryos, those of good quality can be frozen. If the patient conceives successfully on her fresh transfer and has a child, she has the opportunity to have more children by having her frozen embryos thawed and transferred to her uterus in the future. If she does not conceive with transfer of the fresh embryo(s), thaw and transfer of frozen embryos will provide another chance for pregnancy.
If a woman is young and has many eggs or if for women with Polycystic Ovary Syndrome (PCOS), we recommend a “Freeze All” cycle. For women with many eggs we have discovered that pregnancy rates are higher when we do not perform an embryo transfer during the fresh IVF cycle. This is related to the woman making many eggs during the IVF cycle and her body is under stress. In this scenario, we let the woman’s body return to normal and perform the FET in a subsequent cycle, with excellent pregnancy rates.
If on the fresh IVF cycle, ultrasound of the woman’s uterine lining shows that it does not appear to be of adequate thickness to provide a good chance for conception, it will often be recommended that transfer be deferred and all good quality embryos be frozen. At a later date, once the uterine lining is seen to be more favorable on either a natural cycle or in a cycle in which medications such as letrozole or estrogen are given, embryos can be thawed for transfer.
If a patient or couple would like to have a family in the future, but not at the present time, they may choose to proceed with IVF and freeze all their embryos for a later transfer.
If a patient or couple plan to have preimplantation genetic screening (PGS) done to test for chromosomal abnormalities, the embryos are biopsied at the blastocyst stage (day 5 or 6 after egg retrieval) by removing two to three cells from the embryo and submitting this tissue to the genetic laboratory for testing. The biopsied embryos are frozen immediately after the biopsy has been performed. Results of the biopsy will be reported by the genetic testing laboratory for each of the biopsied embryos and the chromosomally normal embryos can then be thawed in the future for transfer to the woman’s uterus.
If a patient has been diagnosed with cancer that will require chemotherapy that may affect the quality of her eggs, she may be able to proceed with IVF and freeze all her embryos from that cycle. Once she is in remission from her disease, she may be able to thaw embryos for transfer to herself or to a gestational surrogate.
In the human body, an egg is fertilized by sperm in the woman’s fallopian tube and the resulting embryo goes through several stages of development as it travels toward the uterus. If an embryo is healthy it will reach the blastocyst stage (approximately 100 – 200 cells) as it enters the uterus from the tube, and is capable of hatching and implanting into the uterine lining. All embryos are in a shell called a zona pellucida and must hatch out of this shell to implant. One reason a woman may not conceive on a natural cycle is that the embryo may be abnormal and therefore stops developing before it reaches the uterus.
After an egg is fertilized in the IVF laboratory, the embryo must proceed through all the same stages as if it were in the woman’s fallopian tube. If an embryo is abnormal, it most often stops developing in the petri dish. The embryos that reach blastocyst (5 to 6 days after the egg retrieval) have the best chance of surviving the thaw and implanting in the woman’s uterus and becoming a viable pregnancy.
Therefore, only the embryos that reach the blastocyst stage are frozen and stored for future thaw and transfer. The freezing process used at Northern California Fertility Medical Center is vitrification, as this has been shown to result in the best chance for pregnancy after the embryos are thawed. At the time of thawing, the embryologists will perform assisted hatching on the embryos selected for transfer by making a tiny opening in the shell of the embryo with a laser. This provides the embryo a better chance of hatching out of the shell (which it must do in order to implant into the uterine lining) and of resulting in a viable pregnancy. Sometimes a blastocyst will hatch on its own upon thawing and will not need the assisted hatching procedure.
Thawed embryos are transferred into a woman’s uterus at the time the uterine lining is most receptive for embryo implantation. This is determined by observation of the lining thickness and pattern at pelvic ultrasound. The physician will set up a plan for either a natural cycle transfer, mildly stimulated cycle transfer (e.g. with the oral medication letrozole), or a replacement cycle transfer. In a replacement cycle, the woman will be taking oral, vaginal, or intramuscular estrogen and progesterone (or a combination of these) in order to prepare her lining. The physician will explain why he or she feels the recommended plan will give that particular patient the best chance for pregnancy.
In all of these types of FET cycles, once the lining is deemed optimal by ultrasound, the date of the thaw and transfer of an embryo or embryos will be scheduled. The patient is offered valium to relax her body for the transfer, or she may opt for no medication. There is very little discomfort from an embryo transfer. The patient will need a full bladder as placement of the catheter for embryo transfer is monitored by abdominal ultrasound, to be sure the catheter tip is seen within the uterus. Once the embryo(s) are placed into the uterus, the patient can return home shortly thereafter.