The preparation of the uterus for a frozen embryo transfer (FET) can vary significantly depending on the approach taken: a natural menstrual cycle, FSH ovulation induction, or hormone replacement therapy (HRT). Each method aims to optimise the uterine lining (endometrium) for successful embryo implantation. Below is a detailed overview of each preparation method.

Uterine Preparation for Frozen Embryo Transfer

1. Natural Menstrual Cycle

In a natural cycle, the body’s own hormonal signals are utilised to prepare the endometrium. The process typically involves the following steps:

• Monitoring Follicle Development: The fertility clinic will monitor the growth of ovarian follicles through ultrasound. This helps determine the timing of ovulation.
• Ovulation Trigger: Once a dominant follicle reaches an appropriate size, an HCG injection may be administered to trigger ovulation. This usually occurs around day 14 of the menstrual cycle.
• Endometrial Development: After ovulation, the body naturally produces progesterone, which is crucial for the maturation of the endometrium. The embryo transfer is scheduled approximately 5 days after ovulation, coinciding with the optimal receptivity of the endometrium.
• Supportive Medications: Progesterone may be supplemented via injections or vaginal suppositories during the luteal phase to ensure the endometrium is adequately prepared for implantation.

2. FSH Ovulation Induction

In cases where natural ovulation may not be reliable, FSH (Follicle Stimulating Hormone) can be used to induce ovulation. The preparation process includes:

• FSH Administration: FSH injections are given to stimulate the ovaries to produce multiple follicles. This is monitored through regular ultrasounds and blood tests to assess hormone levels.
• Ovulation Trigger: Similar to the natural cycle, once follicles are mature, an hCG trigger is administered to induce ovulation.
• Endometrial Preparation: After ovulation, progesterone is provided to support the endometrium. The embryo transfer is scheduled about a week post-ovulation, aligning with the endometrium’s optimal receptivity.
• Monitoring: Throughout this process, the endometrial lining is monitored via ultrasound to ensure it reaches the desired thickness before the transfer.

3. Hormone Replacement Therapy Cycle

In a hormone replacement cycle, the preparation is more controlled and involves the administration of hormones to mimic the natural cycle. This method is often used for women with irregular cycles or those who do not ovulate. The steps include:

• Estrogen Administration: Starting on cycle day 1, estrogen is given in the form of pills, patches, or injections to stimulate the growth of the endometrial lining. This phase typically lasts about two weeks.
• Monitoring Endometrial Growth Regular ultrasounds are conducted to assess the thickness of the endometrium. Once the desired thickness is achieved, progesterone is introduced.
• Progesterone Supplementation: Progesterone is administered (via injections or vaginal forms) starting about 5-7 days before the scheduled embryo transfer. This hormone is crucial for preparing the endometrium for implantation.
• Transfer Timing: The embryo transfer is scheduled approximately 6 days after the start of progesterone, ensuring that the endometrium is at its most receptive state.

The preparation of the uterus for a frozen embryo transfer is a critical step in the in vitro fertilisation (IVF) process. Each method (natural menstrual cycle, FSH ovulation induction, and hormone replacement therapy) has distinct protocols tailored to the individual’s hormonal environment and reproductive health. Understanding these methods can empower patients to engage more fully in their fertility journey and enhance their chances of successful implantation and pregnancy.

Sources:

1. Planning and Preparing for Your Embryo Transfer, Kindbody (2022).
2. Frozen Embryo Transfer, IVF Australia (N.D).
3. Frozen Embryo Transfer, Life fertility Clinic (N.D).
4. Frozen Embryo Transfer Cycle, UW MEDICINE | PATIENT EDUCATION (2016).
5. FET-IVF: Frozen Embryo Transfer Tips, Timelines and Beyond (2024).