Frozen Embryo Transfer (FET)

What is a Frozen Embryo Transfer?

Today, many clinics advise patients to undergo a “freeze-all” cycle of IVF rather than transferring a fresh embryo after retrieval and fertilization. Several contributing factors make the treatment practice of freezing and thawing embryos before transfer a preferred option for many.

Advancements in Freezing

Embryo freezing technology has advanced dramatically over the years. Embryos are now frozen via a process called vitrification. Vitrification is an ultra-rapid process that prohibits the formation of ice crystals. The method also uses cryoprotectants, substances that protect tissue from damage during freezing. Before using vitrification, a slow freeze process was used and often compromised the egg or embryo’s quality. Today, vitrification has minimal (if any) impact on the embryo.

The Uterine Environment

Clinicians have had concerns about the status and uterine receptivity immediately following a stimulated retrieval. Medications used in IVF stimulation result in a highly estrogenic uterine environment, which may accelerate the development of the uterine lining to be ahead of the embryo’s developmental stage — resulting in a lower implantation rate.
Several large IVF centers have recently suggested that freezing all embryos upfront on day 5 (blastocyst) rather than transferring them during a stimulated cycle may improve IVF outcomes. Some programs report higher pregnancy and birth rates in cycles where there is a pause between the retrieval and the transfer.

Allowing for Genetic Screening or Testing

Another important reason for “Freeze All” IVF cycles is the preference for many women and couples undergoing IVF to perform pre-implantation genetic diagnosis or preimplantation genetic screening on their embryos before selecting one for transfer.
Patients may choose to have embryo testing to confirm that the embryo has a normal number of chromosomes or see if their embryos contain a specific genetic mutation that can result in disease.
Testing of the embryo is done by taking a small biopsy on day 5, at the blastocyst stage. A few cells that would become the placenta (trophectoderm) are removed and tested.
The embryos are frozen after the biopsy and are later transferred once the results are available, confirming the embryo’s health being transferred and decreasing the chance of miscarriage.

An Overview of Frozen Embryo Transfers (FET)

Most embryos are frozen on day five at the blastocyst stage by a process called vitrification. This is rapid freezing in liquid nitrogen, which prevents ice crystals, which can be damaging to the embryo. A frozen cycle with day three embryos is similar, with two days less progesterone exposure before transfer.

Success Factors

  • Age of the woman
  • Quality of the embryos
  • Stage of development at which they were frozen
  • The quality of the uterine lining
  • The IVF lab’s embryo freezing protocol

(Always check the IVF programs success rates for fresh and frozen via SART)

Uterine Evaluation

Before starting an FET cycle, the uterine cavity must be evaluated using one of these tests:

  1. Hysterosalpingogram (HSG)- Contrast is injected into the uterus, and the cavity is viewed by x-­ray.
  2. Saline Infused Ultrasound (SIS)- Saline is injected into the uterus and followed by ultrasound.
  3. Hysteroscopy -­ A fiber optic scope is introduced into the uterus, and the cavity is viewed directly. (If any abnormalities are detected, they should be corrected surgically before going forward with a FET cycle).

Protocols for FET

  1. Lupron suppression: Subcutaneous Lupron injections are used for pituitary suppression for approximately 2-3 weeks to prevent unexpected ovulation. Estrogen medications are used to stimulate the growth of the uterine lining and prepare it for implantation. When the lining has reached about 8mm in thickness, Lupron is discontinued, and progesterone supplementation begins. Progesterone prepares the uterus for implantation. Embryo transfer is usually done around the 5th day of progesterone, depending on which day after fertilization the embryo was frozen.
  2. No Lupron suppression: Just as in the above protocol, it uses about 2-3 weeks of estrogen supplementation followed by progesterone once the lining measures 8 mm in thickness. The protocol chosen is based on your specific need. This does not include Lupron.

**Hormone monitoring and ultrasounds are done periodically throughout the cycle.