Frozen Embryo Transfer (FET): Everything You Need to Know

What Is a Frozen Embryo Transfer?

A frozen embryo transfer (FET) is an IVF cycle where previously frozen embryos are thawed and transferred to the uterus. Unlike a fresh transfer where embryos are transferred in the same cycle as the egg retrieval FET separates the stimulation and transfer processes into different cycles. This approach has become increasingly popular and now accounts for the majority of embryo transfers in many leading IVF centres worldwide.

Why Frozen Transfers Are Often Preferred

Multiple studies have demonstrated that frozen embryo transfers often achieve equal or better outcomes compared to fresh transfers. The primary reason is that in a fresh cycle, the high levels of hormones used during stimulation can create a sub-optimal uterine environment for implantation. In a FET cycle, the uterus has had time to recover and return to its natural state, creating a more receptive environment for the embryo.

The FET Process: Step by Step

A FET cycle begins with uterine preparation, which can be done in two ways: a natural cycle (monitoring the natural ovulation and timing transfer around it) or a medicated cycle (using oestrogen to build the uterine lining and progesterone to prepare for transfer). The endometrial lining is monitored by ultrasound until it reaches the optimal thickness typically 7mm or more with a trilaminar pattern. Once ready, embryos are thawed, survival is confirmed, and transfer proceeds. The two-week wait then follows before a pregnancy test.

Embryo Survival After Thawing

Modern vitrification (ultra-rapid freezing) has dramatically improved embryo survival rates after thawing typically exceeding 90% for blastocyst-stage embryos. This means the vast majority of frozen embryos survive the thaw in excellent condition and are suitable for transfer. Embryo survival and quality after thawing is assessed on the morning of transfer by the embryologist.

FET Success Rates and What to Expect

FET success rates are broadly comparable to fresh transfer rates and in many patient populations are higher. As with all IVF outcomes, age and embryo quality are the primary determinants. Blastocyst-stage embryos (day 5–6) generally have higher implantation rates than cleavage-stage embryos (day 3). Patients should discuss with their clinic whether a natural or medicated FET protocol is more appropriate for their individual situation.