In vitro fertilization (IVF) treatment involves stimulating the ovaries, collecting the eggs, fertilizing them in the laboratory to create embryos, and finally transferring fresh embryos into the woman's uterus three or five days after egg retrieval.

 

In frozen embryo transfer (FET) cycles, the embryos were created and frozen in a previous cycle—sometimes a month earlier or even several years ago. When the time comes, the embryos are thawed and transferred into the uterus, which has been prepped by preparing the uterine lining (endometrium).

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Whether it's a fresh or frozen transfer, the endometrium (the lining of the uterus) must be ready for transfer to ensure the embryo can implant and the pregnancy can continue healthily.

 

To assess whether the endometrium is of suitable thickness and quality for transfer, ultrasound monitoring is typically required. Generally, an endometrial thickness of 7 mm is considered appropriate for transfer.

 

In fresh transfer, the estrogen and progesterone hormones produced by the ovarian follicles help prepare the endometrium. In frozen transfer, patients require estrogen and progesterone hormone therapy (hormonal cycle) to prepare the endometrium for implantation. In some cases, the transfer may be done without any medication, during the natural ovulation period (natural cycle).

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Reasons for recommending or performing frozen embryo transfer (FET):

1. Excess embryos from a fresh cycle: If there are excess embryos after a fresh cycle, they can be frozen for future use if pregnancy is desired later. In this case, a frozen embryo transfer will be performed.

2. Excessive hormonal response during IVF: If the hormonal treatment during IVF results in an excessive response, leading to high hormone levels, it may disrupt the endometrium’s readiness for transfer. In such cases, the transfer is canceled, and the embryos are frozen. The transfer will occur in a later cycle when the endometrium is ready or hormonally prepared.

3. Premature ovulation before egg retrieval: If ovulation occurs prematurely during the treatment cycle and progesterone levels rise, the embryo's interaction with the endometrium may be disrupted. In such cases, the eggs are retrieved, embryos are created and frozen, and the transfer is done in a later cycle when the endometrium is ready or hormonally prepared.

4. PGT (Preimplantation Genetic Testing): If PGT is planned before transfer, the embryo is biopsied, and the results take longer than the optimal window for transfer. The embryos are frozen until the results are received, and the transfer will occur in a later cycle when the endometrium is ready or hormonally prepared.

5. Surgical procedures: If surgeries like polypectomy, myomectomy, septum removal, or adhesiolysis are planned after IVF treatment to avoid time loss, the embryos are frozen. After the surgery, the endometrium is allowed to heal, and the frozen embryo transfer is performed later.

6. Thin endometrium at the time of fresh transfer: If the endometrium is not thick enough (less than 7 mm), transferring an embryo may reduce the chances of a healthy pregnancy. In such cases, the embryo is frozen, and the transfer will occur in a later cycle when the endometrium is ready or hormonally prepared.

7. OHSS (Ovarian Hyperstimulation Syndrome): In cases where there is a high risk of developing OHSS, a rare but significant complication of IVF, the embryos are frozen. After the patient recovers from OHSS, a frozen embryo transfer is performed in a later cycle.