Endometrial Thickness Measurement Protocols

The type of endometrial preparation was determined according to the experience of the physician, based on patients’ characteristics, including natural cycle (NC), hormone replacement therapy (HRT) and GnRH agonist combined with HRT (GnRH agonist-HRT) protocol. In short, women with regular menstrual cycles were allocated to NC, while patients having irregular cycles were offered either HRT or GnRH agonist-HRT protocol. The detailed protocols for endometrial preparation were described in our previous studies (23 (link)). To guarantee the accuracy and reliability as possible, EMT was measured by highly trained and experienced sonographers of the same team via Voluson E8 (GE Healthcare, Australia) with intracavity probes. We identified EMT by the largest diameter from one endometrial–myometrial interface to the other in the midsagittal plane. In NC cycles, EMT was measured on the day of hCG administration, while in women with HRT or GnRH agonist-HRT protocol, EMT was recorded from the last ultrasound prior progesterone initiation. Patients were categorized into three groups according to EMT: ≤7.5 mm, 7.5-12 mm and >12 mm, and 7.5-12 mm served as a reference group. These thresholds were selected based on the previous studies (12 (link), 14 (link), 24 (link)).