A Frozen Embryo Transfer (FET) involves thawing a cryopreserved embryo from a prior IVF cycle and transferring it into the uterus to attempt pregnancy. While some FET cycles use hormone replacement therapy to prepare the uterus, a natural cycle FET integrates the body’s own hormonal rhythms with some medical support. A modified natural cycle FET is a variation of the natural FET, incorporating minimal medication to support and optimize the process.
Core Principles of Modified Natural Cycle FET
The modified natural cycle FET leverages the body’s menstrual cycle to prepare the uterine lining for embryo implantation. Unlike fully programmed cycles that rely heavily on exogenous hormones, this approach works with the body’s natural processes. It uses minimal medication to enhance and control the timing of natural events, typically involving close monitoring of follicular development and endometrial thickening through blood tests and ultrasounds.
The “modification” includes a trigger shot, such as human chorionic gonadotropin (hCG), to precisely time ovulation. This ensures predictable ovulation, aiding embryo transfer scheduling. While the body produces its own progesterone after ovulation, minimal progesterone supplementation might be added to support the uterine lining. The goal is to create a uterine environment that closely mimics a natural conception, reducing the medication burden.
The Procedure Explained
The modified natural cycle FET begins with assessments to confirm a patient’s regular menstrual cycle and suitability. Once the cycle starts, regular monitoring is crucial. This involves transvaginal ultrasounds and blood tests, usually starting around day 8-12, to track the growth of a dominant follicle and the thickening of the uterine lining (endometrium). The aim is for the endometrium to reach at least 7 millimeters and the dominant follicle 16-18 millimeters.
When the follicle and endometrial lining reach desired parameters, a trigger shot of hCG is administered to induce ovulation. This typically causes ovulation to occur approximately 36 hours later. The timing of the embryo transfer is precisely calculated based on this induced ovulation. For a day 5 or 6 blastocyst, transfer usually occurs 5 to 7 days after the trigger shot.
Following embryo transfer, some clinics may recommend minimal progesterone support, often in the form of vaginal suppositories, to support the uterine lining during the luteal phase. This support typically continues until a pregnancy test is performed, and if positive, may extend for several more weeks. The entire process is designed to align the embryo transfer with the body’s natural window of implantation.
Patient Suitability
Modified natural cycle FET is for patients with regular menstrual cycles and predictable ovulation. These individuals have healthy ovulatory function, naturally producing the hormones necessary for endometrial preparation. The approach is appealing to those minimizing the use of exogenous hormones and preferring a more physiological method for embryo transfer.
This protocol is suitable for patients with a history of spontaneous pregnancies now facing infertility due to factors like tubal issues or severe male factor infertility. The reduced reliance on external medications can lead to fewer side effects and a less stressful experience compared to fully programmed cycles. Success rates are generally comparable to other FET protocols for appropriate candidates.
Important Considerations
Modified natural cycle FET requires intensive monitoring. Due to reliance on the body’s natural cycle, frequent ultrasound scans and blood tests track follicular development, endometrial thickness, and hormone levels. This ensures optimal timing for the trigger shot and embryo transfer.
Cycle cancellation is possible if ovulation doesn’t occur as expected or the uterine lining isn’t optimal. This can be emotionally challenging for patients who have invested time and hope into the cycle. While the goal is to reduce medication, precise timing means less flexibility in scheduling compared to fully programmed cycles. Live birth rates in modified natural cycles are comparable to programmed cycles for suitable candidates.