Freezing of Embryos at Northern California Fertility Medical Center
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.
Patient Preparation and Procedure for Frozen Embryo Transfer
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.
