What Is a Natural Cycle FET With a Trigger Shot?

A natural cycle frozen embryo transfer (FET) with a trigger shot is a fertility treatment that aligns the transfer of a frozen embryo with the body’s natural menstrual cycle. This approach relies on the body’s own hormonal cues to prepare the uterine lining, with the goal of placing the embryo into the uterus when it is most receptive. The addition of a “trigger shot” provides control, ensuring the timing of the procedure is optimized for the highest chance of implantation.

Understanding the Natural Cycle FET Approach

The “natural cycle” in a Frozen Embryo Transfer refers to a protocol that works with a person’s own reproductive rhythm. Instead of using medications to suppress hormones and build the uterine lining, this approach monitors the body’s spontaneous development of a follicle and the thickening of the endometrium. This process allows the body to prepare for a potential pregnancy as it would in a cycle without fertility treatment.

Monitoring is a central component of this approach. Fertility specialists use transvaginal ultrasounds and sometimes blood tests to track the growth of the dominant follicle. As this follicle grows, it releases estrogen, which naturally causes the uterine lining, or endometrium, to thicken and prepare for implantation. The objective is to identify the ideal window for the embryo transfer.

This method contrasts with a medicated or programmed FET, where medications like estrogen and progesterone are used to artificially prepare the uterine lining. This gives the clinic complete control over the timing of the transfer. The natural cycle approach relies on the patient’s own hormonal system to regulate the process, reducing the number of hormonal medications required.

The Significance and Timing of the Trigger Shot

A “trigger shot” is an injection of a hormone, such as human chorionic gonadotropin (hCG) or a GnRH agonist. In a natural FET cycle, its purpose is to induce the final maturation of the egg and prompt ovulation at a specific time. Even though the released egg is not intended for fertilization, this process signals the body to produce progesterone, which makes the uterine lining receptive to an embryo.

The primary role of the trigger shot is precision. While the cycle develops naturally, the trigger provides control over the timing of ovulation. This allows the fertility clinic to schedule the embryo transfer for the precise moment when the uterine lining will be optimally prepared. This coordination is meant to mirror the natural synchronization between ovulation and embryo arrival in the uterus.

Determining when to administer the trigger shot is based on careful monitoring. Ultrasounds are used to measure the lead follicle, and ovulation is triggered when it reaches a diameter of about 16 millimeters or more. Simultaneously, the thickness of the uterine lining is assessed, with a minimum of 7 millimeters often desired. Blood tests for luteinizing hormone (LH) may also be used to confirm the body is approaching its natural surge.

The Natural Cycle FET Procedure with Trigger Explained

The process begins with a baseline ultrasound near the start of the menstrual cycle to ensure there are no cysts and the ovaries are quiet. Following this, monitoring starts, involving several appointments for transvaginal ultrasounds to track the growth of the dominant follicle and measure the thickening of the endometrial lining. Blood tests may also be conducted to check hormone levels.

Once monitoring shows the lead follicle has reached an adequate size and the uterine lining is sufficiently thick, the trigger shot is self-administered at home. The shot initiates the final processes of ovulation, which is expected to occur about 36 hours later. This timing allows for the exact scheduling of the embryo transfer.

The embryo transfer is scheduled for a specific number of days after the trigger shot, based on the embryo’s developmental stage. For instance, a blastocyst, an embryo that has developed for five days, is transferred five days after ovulation has occurred. On the day of the procedure, the selected embryo is thawed and transferred into the uterus using a thin, flexible catheter.

Following the transfer, luteal phase support is often prescribed. This involves taking progesterone supplements to ensure the uterine lining remains stable and receptive. This extra support is often provided to support the early stages of a potential pregnancy. A pregnancy test, usually a blood test measuring hCG levels, is performed around 9 to 14 days after the transfer.

Determining Suitability for This Protocol

This protocol is a strong option for individuals who have regular and predictable menstrual cycles. Because the procedure relies on the body’s natural ability to select and mature a follicle, consistent ovulation is a primary requirement. Patients who have previously demonstrated the ability to develop a thick uterine lining without hormonal medication are also considered excellent candidates.

Factors that contribute to a person’s suitability include a cycle length that is reliably between 21 and 35 days. The predictability of these cycles is a significant advantage, as it makes the monitoring phase more straightforward. The reduced medication load is often seen as a benefit, both in terms of physical side effects and financial cost.

Conversely, this approach may not be suitable for everyone. Individuals with a history of anovulation (a lack of ovulation) or highly irregular menstrual cycles would likely benefit more from a medicated cycle that offers greater control. If a patient has a history of a persistently thin endometrium in unmedicated cycles, a programmed protocol with estrogen support may be recommended to ensure the lining is adequately prepared.