FROZEN EMBRYO TRANSFER (FET)
FET treatments can be carried out either during a hormonally supported cycle or in a natural cycle.
Hormonally Supported FET Cycle
This is particularly helpful if your periods are very erratic or infrequent. Your pituitary gland is first "down-regulated" as in IVF (see IVF information) using daily injections or a nasal spray. Once down-regulation has been achieved and ultrasound scanning shows very thin endometrium, the process of building up the endometrium is commenced. To achieve this, you will take oestrogen hormone tablets as well as continuing to self-administer the daily down-regulation treatment, until the endometrial thickness is adequate on scan.
If it is considered that the endometrium is too thin, there is no question of thawing out embryos and wasting them in a cycle where there is no chance of implantation and pregnancy. The embryos will wait in storage until the endometrium is ready. The oestrogen dosage may need to be increased in order to achieve this.
Once the endometrium is satisfactory, the daily down-regulating injections are stopped. While continuing to take the oestrogen tablets, you will now commence using progesterone vaginal pessaries or cream to support the endometrium lining and hopefully encourage implantation.
Embryo thawing is timed so that FET is usually carried out after you have been on the progesterone for three days.
The timing of embryo thawing will depend upon the stage of development reached when cryopreservation was carried out. Embryos that have been frozen shortly after fertilisation at the so-called 2PN stage, are thawed the day before the planned FET, whereas more advanced (cleaved) embryos are thawed out on the day of FET.
The embryos are stored in vials, as single embryos, or in twos. It may happen that all the embryos you have in store will need to be thawed in order to obtain up to two that are suitable for transfer. Sadly, sometimes none of the embryos survive the freezing and thawing out process.
As in normal IVF, the maximum number of embryos that can be transferred is two unless you are over 40 years of age, when in exceptional circumstances three embryos may be transferred. However, transferring three embryos does run the increased risk of a multiple pregnancy.
FET is identical to fresh IVF embryo transfer. During the week after FET you are advised to lead a gentle existence if at all possible.
The oestrogen hormone tablets and progesterone preparation are continued either until a period occurs or until a pregnancy is well established.
Natural Cycle FET
This is a very straightforward process. From the pattern of your previous cycles the centre will know the most likely timing of the LH (luteinising hormone) surge that occurs 24 to 36 hours before spontaneous ovulation. If your periods are moderately irregular, ovulation can usually be made into a predictable event using fertility drugs e.g. clomifene. From about three days before the expected LH surge, you will be have either blood tests or may be asked to use an ovulation predictor kit on a morning urine sample each day.
On the day that blood or urine testing indicates the LH surge, the centre will contact you and arrange to see you in the next 24 to 48 hours to carry out a vaginal scan to determine whether or not the endometrium lining of the uterus has developed adequately. As in hormonally supported FET, the guiding principle is that the embryos will wait in storage until the endometrium is ready. Sometimes it can happen that the endometrium is inadequate in spite of there being an LH surge. In such cases the cycle would be abandoned and a future hormonally supported cycle would be planned.
As in hormonally supported cycles, the timing of embryo thawing will depend upon the stage of development reached when cryopreservation was carried out. 2PN embryos are thawed two days after the LH surge with a view to transfer the following day (LH surge + 3 days). Cleaved embryos are thawed out on the day of embryo transfer.
For sixteen days after FET you will be asked to use the vaginal progesterone preparation.