The rise in frozen embryo transfer (FET) cycles within assisted reproductive technology (ART) has sparked the need to refine endometrial preparation protocols. The switch from hormone replacement cycles (HRT) to natural cycles, poses scheduling challenges onto laboratories, doctors and patients. The modified natural cycle (mNC), offering more precision for scheduling, shows promise but lacks comprehensive data on the potential flexibility of triggering criteria that may allow to control timing in a wider range.

This prospective randomized study seeks to address this knowledge gap by evaluating the non-inferiority of triggering ovulation in mNC with follicle diameters of 13-16.9mm compared to the conventional 17-22mm.

The study aims to enhance the understanding of the impact of follicle size at ovulation in reproductive outcomes, optimize FET treatments providing a more flexible mNC protocol, and improve patient care in ART clinics.

Inclusion Criteria:

• At least one expanded blastocyst (regardless of its quality)
• Spontaneous menstrual cycles
• Consent to undergo frozen embryo transfer in a modified natural cycle

Exclusion Criteria:

• Abnormal uterine cavity
• Fluid in endometrial cavity

All IPD collected throughout the trial.