Fresh Embryo Transfer vs Frozen Embryo Transfer (FET) As one of the most important decisions in assisted reproductive technology (ART), the choice between a Fresh Embryo Transfer vs Frozen Embryo Transfer (FET) can be pivotal. Both procedures are intended to help couples who are having infertility issues, but they are done in different ways and at different times. Here’s a breakdown of how the two methods compare, including their processes, advantages, challenges, and results. 

1. Definition of Fresh Embryo Transfer and Frozen Embryo Transfer (FET) 

• Fresh Embryo Transfer: A fresh embryo transfer is done in the same cycle as the egg retrieval procedure. The eggs are fertilized in the lab, and the resulting embryos are placed into the woman’s uterus days later. During this process, the embryos are not frozen and are directly transferred after fertilization. 
• Frozen Embryo Transfer (FET): A frozen embryo transfer is the transfer of embryos that were produced in a previous IVF (In Vitro Fertilization) cycle and subsequently frozen for future use. These embryos are thawed and put back into the woman’s uterus at a later date, such as during a later menstrual cycle or during an “assisted” cycle in which medication is given to optimise receptiveness of the uterus. 

2. Timing of the Procedure 

• Fresh Embryo Transfer: In a fresh embryo, the timing of the transfer is dictated by the ovarian stimulation cycle. After retrieval, fertilization, and culture of eggs have occurred for 3–5 days, embryos of the best quality can be selected for transfer. The transfer usually takes place 3 to 5 days after the egg retrieval. The woman is still using the ovarian stimulation hormones. 
• Frozen embryo transfer (FET) is done on a later day, once the embryos have been frozen and kept. This process is usually planned during a more regulated cycle, where hormone treatment (like estrogen and progesterone) is used to prepare the lining of the uterus for transfer. The timing can be more flexible than a new transfer, and the woman may have more control over her cycle. 
  

3. Hormonal Environment and Effect on Uterus 

• Fresh Embryo Transfer (FET): In fresh transfers, the woman’s body is subjected to high amounts of hormones because of the ovarian stimulation. This can occasionally cause the uterine environment to be less than ideal, especially in cases of the painful condition known as OHSS (ovarian hyperstimulation syndrome), where the ovaries become enlarged and agonisingly painful. Additionally, changes in stimulation hormones can impact the internal environment of the uterus and its potential for implantation. 
• Frozen embryo transfers, the hormone therapy employed may better regulate the woman’s reproductive environment. Having the estrogen and progesterone levels be timed precisely and at the right dosage allows doctors to set the stage for an optimal implantation, which often helps with better success rates. In FET, the ovaries of the women have recovery time from a previous stimulation, which can allow for a more natural uterine environment for implantation of an embryo. 
  

4. Success Rates 

Both fresh and frozen embryo transfers achieve relatively high rates of success, although in the last few years, FET has become associated with marginally higher rates of successful pregnancy. This is mostly due to being able to better control and optimise the uterine environment, along with the embryos being transferred after the woman's body has had time to recover from the stimulation cycle. 

Research demonstrates that instead FETs manage to reduce the rates of complications, including ovarian hyperstimulation syndrome (OHSS), which can occur with fresh transfers, particularly for women with high egg yields. FETs are also, in general, less stressful for the patient because they don’t have to endure the hormonal roller coaster of a fresh IVF cycle. 

5. Benefits of FET 

• Better Uterine Conditions: FETs can be done once the body has recovered and the uterus is prepared in a controlled manner, which leads to a higher success rate compared to fresh transfers. 
• Reduced Risk of OHSS: Because fresh ovarian stimulation is not performed in FET, the chances of encountering the complication known as ovarian hyperstimulation syndrome (OHSS) are minimised. 
• Freezing: In FET the embryos are frozen at their best quality, ensuring the best chance they have of surviving defrosting. This process helps doctors choose the healthiest embryos from the prior cycle. 
• Flexibility: Having the option to freeze embryos from a prior cycle allows for flexibility in future transfers. For example, if a woman is not prepared for the transfer of an embryo or cancels the cycle in progress, the embryos can be banked for future use. 
  

6. Challenges and Considerations 

• Embryo Quality: Not all embryos make it through the freeze/thaw process, affecting success rates. Modern freezing techniques in use today (like vitrification) vastly improve embryo survival rates. 
• Costs: IVF cycle with FET may still cost more than elective FET because the initial IVF cycle needed to retrieve and freeze the embryos must be paid for. In addition, there are storage fees associated with the freezing of the embryos. 
• Embryonic preservation: There is a delicate process involved with thawing frozen embryos, and not all embryos survive this freezing process. However, recent improvements in cryopreservation of oocytes have resulted in even higher thaw survival rates due in part to the introduction of the vitrification technique. 

Conclusion 

Each new cycle triggers a choice between fresh and frozen embryo transfers. While fresh embryo transfers offer the advantage of being performed immediately after egg retrieval, they still carry hormone risks and potential complications. Conversely, thawed embryo transfers create a controlled environment within the uterus, leading to higher implantation success, particularly with new cryopreservation technologies. Whether an FET or fresh transfer is eventually undertaken is very much dependent on the particular patient, her doctor’s advice, and the woman’s response to the IVF. 

To summarise, both the fresh and frozen embryo transfers may lead to a successful pregnancy, however, the gradual process of frozen embryo transfers illustrates better rates of successful pregnancy, hence the increase in use of frozen embryo transfers amongst many couples.