A Frozen Embryo Transfer (FET) is a procedure within In Vitro Fertilization (IVF) that uses an embryo created and cryopreserved (frozen) during a previous IVF cycle. This differs from a fresh transfer, where the embryo is transferred immediately after fertilization and egg retrieval. FET separates the ovarian stimulation phase from the uterine preparation and transfer phase, allowing the body to recover and normalize its hormonal environment. The goal of a frozen transfer is to synchronize the embryo’s developmental stage with the uterine lining’s optimal window of receptivity, maximizing the chances for successful implantation.
Advantages of Using Frozen Embryos
Choosing a Frozen Embryo Transfer optimizes the uterine environment, which is often disrupted during the ovarian stimulation phase of a fresh IVF cycle. High doses of hormones used to stimulate multiple eggs can lead to supraphysiological levels of estrogen and progesterone, hindering synchronization with the embryo’s needs. Delaying the transfer allows the body’s hormone levels to return to a more natural state, mimicking the conditions of a spontaneously conceived pregnancy. Studies also suggest that pregnancies resulting from frozen embryos may have better outcomes, including a lower incidence of preterm birth and healthier birth weights, compared to fresh transfers.
The freeze-all strategy also provides time to perform Preimplantation Genetic Testing (PGT) on the embryos before transfer. PGT involves taking a small biopsy, typically at the blastocyst stage, to screen for chromosomal abnormalities (PGT-A) or specific genetic disorders (PGT-M). Since testing takes several weeks, embryos must be frozen and stored until a genetically screened, or euploid, embryo can be selected for the subsequent FET cycle.
Beyond the physiological benefits, FET offers logistical flexibility for the patient and the clinic. Unlike a fresh transfer, which is strictly timed after egg retrieval, a frozen transfer can be scheduled at a time convenient for the patient’s personal or professional life. This flexibility allows the medical team to address potential concerns, such as a thin uterine lining or the risk of Ovarian Hyperstimulation Syndrome (OHSS). For patients at high risk of OHSS, freezing all embryos and performing a later FET eliminates the risk of pregnancy exacerbating the condition, which might otherwise necessitate cancelling a fresh transfer.
Preparing the Uterus for Transfer
Achieving endometrial receptivity—the state where the uterine lining is ready to receive an implanting embryo—is the focus of the preparation phase, which utilizes two main protocols: medicated and natural cycles. The Medicated, or Hormone Replacement Therapy (HRT), cycle is the most common method. It involves administering external hormones to suppress the natural cycle and precisely control the transfer timing. This protocol typically begins with estrogen supplementation, delivered via pills, patches, or injections, for 10 to 14 days to thicken the uterine lining, aiming for a thickness of at least 7-8 millimeters.
Once the desired endometrial thickness is confirmed via transvaginal ultrasound, the patient begins progesterone supplementation. Progesterone transforms the uterine lining into a receptive environment, opening the “implantation window.” The embryo transfer is precisely timed based on the day progesterone is started, typically five days later for an embryo frozen at the blastocyst stage. This programmed approach offers predictability, making it easier to coordinate the transfer date for the patient and the laboratory.
In contrast, the Natural cycle relies on the patient’s own hormonal fluctuations and requires minimal medication. This protocol is reserved for patients with regular menstrual cycles and involves close monitoring of natural follicular growth and hormone levels, specifically tracking the surge of Luteinizing Hormone (LH) that precedes ovulation. The embryo transfer is timed to coincide with the natural ovulation window. A modified natural cycle may utilize a trigger shot, such as human Chorionic Gonadotropin (hCG), to induce ovulation and improve timing control. Both natural and modified natural cycles benefit from forming a corpus luteum, the temporary endocrine gland that produces progesterone and other hormones supporting early pregnancy, which is absent in fully medicated cycles.
The Frozen Embryo Transfer Procedure
The transfer day begins with the embryology lab carefully thawing the cryopreserved embryo. This process is highly successful, with survival rates often exceeding 90% when embryos are frozen using the modern vitrification (ultra-rapid freezing) technique. After thawing, the embryologist assesses the embryo’s viability and loads the selected embryo into a thin, flexible catheter used for the transfer. The transfer is a quick, non-surgical, and generally painless outpatient procedure that does not require anesthesia.
During the procedure, the physician uses abdominal ultrasound guidance to visualize the uterine cavity and ensure precise placement of the embryo. The catheter is gently passed through the cervix and into the uterus. The embryo, suspended in a tiny amount of culture medium, is released near the middle of the uterine cavity, approximately one to one-and-a-half centimeters from the top (fundus). Precise placement is a factor in the procedure’s success. The patient is advised to have a comfortably full bladder for ultrasound visualization, which helps straighten the angle between the cervix and the uterus.
Following the transfer, patients are advised to rest briefly at the clinic before resuming light, normal activities, as prolonged bed rest has not been shown to improve outcomes. The patient continues taking any prescribed progesterone and estrogen medications to support the uterine lining during the subsequent “two-week wait.” This period ends with a blood test measuring the level of human Chorionic Gonadotropin (hCG), the hormone that confirms pregnancy.
Key Factors Influencing Success Rates
The likelihood of successful implantation and live birth following a Frozen Embryo Transfer is influenced by several variables. The most significant factor impacting success is the quality of the embryo, determined by its chromosomal integrity and developmental stage. Transferring a blastocyst—an embryo developed for five or six days—is associated with higher clinical pregnancy rates compared to earlier-stage embryos. Using Preimplantation Genetic Testing (PGT) to select a euploid (chromosomally normal) embryo further increases implantation chances, largely mitigating the negative effect of advanced maternal age on embryo quality.
Beyond the embryo, the receptivity of the uterine lining is a major determinant of success. Endometrial thickness on the day of progesterone initiation or transfer is a measurable predictor; a thickness of 8 millimeters or greater is associated with better outcomes. Receptivity is also supported by the proper timing and administration of hormones, whether generated naturally or supplied through medication. A well-synchronized uterine environment, a hallmark of the FET process, often results in success rates comparable to or slightly higher than those achieved with fresh transfers.
Other patient-specific factors also play a role, including female age and body mass index (BMI). While embryo quality is the primary driver, older age and obesity (BMI of 25 kg/m² or higher) have been linked to a decreased likelihood of clinical pregnancy in programmed FET cycles. The overall success of the FET procedure is a combination of these elements: the genetic health and developmental quality of the thawed embryo, the thickness and receptivity of the uterine lining, and the precise timing and execution of the transfer.