Frozen Embryo Transfer Endometrial Preparation

Endometrial Preparation Before Frozen Embryo Transfer – Clinical Guide

What is Endometrial Preparation?

It is the process of preparing the inner lining of the uterus (endometrium) to be receptive for embryo implantation before frozen embryo transfer (FET).

Purpose

To achieve an endometrium of optimal thickness (generally ≥7-8 mm), with a trilaminar structure, and receptive.

Preparation Methods

Endometrial preparation is performed using three main methods: natural cycle, modified natural cycle, and hormone replacement therapy (HRT).

1. Natural Cycle

The patient's own ovulation is monitored. After ovulation, it is supported with progesterone, and the transfer is timed.

Advantages

Physiological hormonal environment, lower medication use

Disadvantages

Difficult to apply in irregular cycles

2. Modified Natural Cycle

Follicle tracking is performed, and ovulation is controlled by administering a trigger shot at the appropriate time.

Advantages

Provides more controlled timing

3. Hormone Replacement (Artificial Cycle)

The endometrium is thickened with estrogen, then progesterone is added, and transfer is planned.

Estrogen Use

• Oral, transdermal, or gel form
• Usually started on day 2–3 of menstruation
• Continued until the endometrium reaches 7–10 mm

Initiation of Progesterone

Synchronized with embryo age:
• Day 3 embryo → 3 days progesterone
• Day 5 embryo → 5 days progesterone

Endometrial Monitoring

Thickness and structure are evaluated by ultrasound. Dose adjustments are made if necessary.

Ideal Endometrial Characteristics

• Thickness: ≥7 mm
• Trilaminar appearance
• Good vascularization

Problematic Endometrium

In cases of thin endometrium (<6-7 mm), dose increase, different preparations, or additional treatments (e.g., PRP) may be considered.

Transfer Timing

After progesterone initiation, transfer is performed according to the embryo's developmental day.

Conclusion

Endometrial preparation is one of the most critical steps for FET success. Personalized protocol selection determines success.