When undergoing a frozen embryo transfer (FET), there are two primary methods for preparing the uterus to receive the embryo: the medicated approach and the natural approach. Each method offers a distinct pathway to optimize the chances of a successful pregnancy and understanding both can help individuals make an informed choice. We spoke with Dr. Alexander Kucherov, Reproductive Endocrinologist at Illume Fertility, to help us understand the intricacies.
Medicated FET Cycles
The medicated approach involves the administration of estrogen, which may be given in the form of oral tablets, pills, or injections. This is followed by a diagnostic ultrasound to assess the thickness of the uterine lining, which ideally should measure at least 7 millimeters or greater to ensure optimal receptivity. Once the appropriate uterine lining has been established, the embryo transfer is typically scheduled 7 to 10 days later. In this process, progesterone injections are introduced, typically administered intramuscularly, to transform the uterine lining from a growth phase into a receptive state, preparing it for embryo implantation.
Natural FET Cycles
The natural frozen embryo transfer cycle, by contrast, involves far fewer interventions. This approach follows the natural growth and development of the individual’s follicle throughout the menstrual cycle, with periodic monitoring of the uterine lining at key intervals. Once the follicle reaches the necessary size for ovulation—typically about 16 x 16 millimeters—and the uterine lining has thickened to at least 7 millimeters, a single subcutaneous injection is administered to trigger ovulation. Although the egg released during this process is not used for fertilization, it plays a vital role in stimulating the body’s natural preparation for embryo implantation. The embryo transfer is then scheduled about a week later. In most cases, vaginal progesterone is used to support the uterine lining, but other injections or medications are generally unnecessary.
The natural cycle approach offers numerous benefits, with one of the most significant being the elimination of the need for intramuscular injections, which some individuals find uncomfortable or challenging. Additionally, there is emerging evidence suggesting that the natural cycle may contribute to improved maternal health outcomes. Notably, this approach may reduce the risk of complications such as preeclampsia and hypertension during pregnancy, particularly among individuals over 35 years old. For this reason, the natural cycle is often considered the preferred option for many patients undergoing a frozen embryo transfer.
However, for individuals who experience challenges such as ovulatory disorders (e.g., polycystic ovarian syndrome, or PCOS), achieving a natural cycle may be more difficult. In such cases, a modified natural cycle can be utilized, incorporating medications such as letrozole or Clomid, or even injectable medications, to stimulate follicle development. From this point, the cycle continues in a manner similar to a traditional natural cycle, with monitoring and support provided as needed.
It is important to note that both natural and medicated frozen embryo transfer cycles generally yield similar pregnancy success rates. Given this, it is advisable to consult with a healthcare provider to determine which approach aligns best with an individual’s unique circumstances and reproductive goals. Personalized guidance from a physician can help ensure that the chosen path is the most suitable for optimizing the chances of a successful and healthy pregnancy.