Frozen embryo transfer ( FET ) , is part of an IVF cycle where the embryo (s ) ,whether Day 2/3 embryos or Day 5/6 blastocyst ,which were frozen in the earlier cycle , is thawed and transferred back into a naturally or medically prepared uterus in order to have a baby.
Frozen embryo is a term for embryo(s) that are not transferred during a fresh IVF cycle and deemed suitable by the embryologist to be frozen ( a process of cryopreservation via vitrification ) for future .
Almost 70% of Ever Link patients have at least an embryo frozen from their IVF cycle.
Embryos are frozen for a few reasons and the common ones are :
- An intentional freeze all cycle where there is a risk of OHSS for the patient , any underlying suspicious endometrial lining such as polyps or fibroids which may adversely affect chances of implantation or when there is asynchrony between the endometrium and embryo form hormonal imbalances .
- When there are additional embryos following a fresh transfer , which would be kept for future .
- Donor Programme where the embryos are kept frozen following a Donor IVF cycle while the recipient undergoes her uterus lining preparation for transfer .
- In cases where previous fresh transfer were unsuccessful
- Sometimes , due to patient request.
Prior To FET
It is very crucial to ensure there is no endometrial structural issues such as polyps , adhesions , cervical stricture , cavity distorting fibroids or just for reassurance before replacing any embryo(s) into the cavity .
These embryos are very precious and the whole purpose of an IVF stimulation and undergoing egg collection and the meticulous efforts in the lab to culture embryos up to the level of freezing them will all be wasted if they were replaced into a unknown endometrial cavity .
Sometimes we will advise some form of uterine cavity investigation before embarking onto Frozen embryo Transfer Programme . They could consists of :
- 3D Transvaginal Ultrasound of the uterus
- Hysterosalpingogram (HSG) – which is an X ray procedure done by a radiologist, where a dye is injected into the cavity to visualize for any abnormality which can be seen on the x ray .
- Hysteroscopy – this is a minor procedure to directly visualize the endometrial cavity and treat at the same time correcting any abnormalities. Hysteroscopy is done in an operating theatre and usually takes 15 – 30 minutes and carries very minimal risks. It is normally considered as the ”Gold Standard” investigation of the endometrial cavity .
There are many protocols to prepare the endometroium to make it ideal for implantation.
There is no one protocol superior to another , as long as it can achieve the required endometrial thickness and appearance for a ‘good endometrium ‘ and the protocol are based on specific patient’s gynaecological background .
Common Protocols are :
- Natural Cycle : where no medications are used before the embryo transfer and the cycle is tracked for ovulation using blood tests and serial ultrasound scans to measure the thickness and maturity of the endometrium. This works well for women with regular menstrual cycle.
- Hormone Replacement Therapy ( Medicated Cycle ): with estrogen containing tablets to increase the endometrial thickness ( uterine lining ) or prior treatment with GnRH agonist injection in what we call a ‘down regulation’ before commencing the medicine.
- Stimulated Cycle : with oral medication +/- additional FSH injections to grow follicles akin an IUI cycle and track the egg and endometrium with regular ultrasound scan and hormone blood tests .
Studies demonstrate similar implantation and pregnancy rates regardless of the protocol used for endometrial preparation prior to transfer of frozen–thawed embryos.