A trigger shot, typically an injection of human chorionic gonadotropin (hCG), prepares the body for specific events in fertility treatments. A Frozen Embryo Transfer (FET) involves implanting a previously cryopreserved embryo into the uterus. Precise timing is important for a successful FET, as it helps align the uterine environment with the embryo’s developmental stage.
The Trigger Shot’s Role in FET
In a Frozen Embryo Transfer cycle, a trigger shot, usually containing hCG, prepares the uterine lining. This differs from its role in fresh In Vitro Fertilization (IVF) cycles, where it primarily induces final egg maturation and ovulation for retrieval. For FET, the hCG injection mimics natural hormonal signals that ready the endometrium, the inner lining of the uterus, for embryo implantation. This preparation supports uterine receptivity, creating an environment conducive for the embryo to attach and grow. The trigger shot helps synchronize uterine readiness with the embryo’s developmental stage before transfer.
Pinpointing the Transfer Timeline
General Timing
The exact timing of a Frozen Embryo Transfer after a trigger shot depends on the embryo’s frozen stage and the specific clinic protocol. In natural or modified natural FET cycles, the trigger shot induces ovulation, typically 36 hours post-injection. Progesterone administration usually begins afterward, and the embryo transfer is then timed based on the number of days of progesterone exposure.
Embryo Stage Considerations
For blastocyst-stage embryos (Day 5 or Day 6), transfer generally occurs after 5 to 6 days of progesterone support. If the embryo was frozen at an earlier cleavage stage (a Day 3 embryo), transfer might be scheduled after 3 to 4 days. The total days from trigger shot to transfer encompass the time until progesterone initiation plus subsequent progesterone exposure, aligning with the embryo’s development. This synchronization aims to optimize the window of implantation, the period when the uterus is most receptive to an embryo.
Preparing for the Embryo Transfer
Monitoring Uterine Readiness
After the trigger shot and leading up to the embryo transfer, careful monitoring ensures optimal uterine preparation. This involves regular transvaginal ultrasound scans to assess endometrial thickness and pattern. An endometrial thickness of at least 7 millimeters, ideally between 7-10 millimeters, is favorable for implantation. The lining’s “trilaminar” or three-layered appearance on ultrasound also indicates receptivity.
Hormonal Support
Blood tests monitor hormone levels, particularly estrogen and progesterone. Estrogen thickens the uterine lining, while progesterone matures and stabilizes it, making it receptive. Progesterone initiation timing is carefully controlled, as transfer is scheduled a specific number of days after exposure begins. Patients receive detailed medication instructions to ensure the uterine environment is properly prepared for the embryo.
The Transfer Procedure and Post-FET Care
The Transfer Procedure
The embryo transfer procedure is typically non-surgical, involving careful placement of the thawed embryo into the uterus using a thin, flexible catheter, usually guided by ultrasound imaging. Patients are often advised to have a full bladder for better visualization during the procedure.
Post-Transfer Care
Following the transfer, patients enter the “two-week wait” before a pregnancy test can be reliably performed. During this time, continued medication, primarily progesterone, supports the uterine lining and potential early pregnancy. Progesterone supplementation typically continues until the placenta takes over hormone production, usually around 8-10 weeks of gestation. A blood test for human chorionic gonadotropin (hCG), the pregnancy hormone, is usually scheduled 10 to 14 days after the embryo transfer to confirm pregnancy.