Frozen Embryo Transfers (FET)
Frozen Embryo Transfer
Following IVF, excess embryos are frozen for use with second attempts if no pregnancy takes place or to conceive a second child. With improvement of the freezing and thaw techniques: the majority of frozen embryos survive thawing, the implantation potential of a thawed embryo is comparable to a fresh embryo, less embryos or single embryo can be transferred in the fresh cycle and selection of the best embryo for fresh transfer became less important.
Freezing of embryos allow ample time for genetic testing of embryos if needed, transferring embryos to a different locale, delaying transfer due to medical problem, the emergence of an abnormality in the lining of the uterus e.g thin endometrium, polyp, fluid.. or till a gestational carrier is found.
Benefits of Frozen Embryo Transfer
1. Pregnancy rate after frozen embryo transfer is comparable to fresh transfer and may even be higher than fresh transfer in some studies. More work is needed to confirm higher live birth rate.
2. Complications: frozen embryo transfer minimize some of the complications related to IVF. Ovarian hyperstimulation syndrome (OHSS) and possibly ectopic pregnancy (pregnancy in the fallopian tube)
3. Lower risk for pregnancy complications and better quality baby: frozen transfer appear to reduce the risk for preterm delivery, bleeding in pregnancy and low birth weight, possibly due to better placental function.
How is the lining of the uterus prepared for frozen embryo transfer?
1. Natural cycle: in ovulating women, the follicle in the ovary is monitored till the point of ovulation is accurately identified. The follicle will internally produce the estrogen required to build the lining. When ovulation takes place, the embryos are thawed and transferred in a day comparable to its age e.g a day 5 embryo is transferred 5 days after ovulation. This process require only ultrasound and blood work monitoring
2. Estrogen replacement cycle: ovulation is stopped and estrogen is supplemented externally (patches,oral or vaginal) till the desired thickness and pattern of the uterine lining. Progesterone is then started (injection or vaginal) then embryos are transferred.
Timing of thaw and transfer is a complicated question and it depends on the type of cycle and age of embryos. Sometimes embryos are thawed and cultured for few days before transfer
All method for endometrium preparation yield similar pregnancy rate. At NYCIVF we prefer natural cycle with luteal phase support using vaginal estrogen.
What makes a frozen embryo transfer cycle successful?
Embryo quality: one or more top quality embryo morphology observed at any stage of culture improves the outcome even if high-quality characteristics disappeared before transfer. Transferring more than one embryo increases the pregnancy rate but also multiple pregnancy.
Conclusion: should you intentionally delay transfer to frozen cycle? no but if you need to freeze the embryos, expect similar pregnancy rate as in the fresh cycle.