Fig. 2

Endometrial preparation protocol and follow-up. Hormone replacement therapy patients were started on estradiol from day 2 to day 5 of the menstrual cycle. Endometrial thickness was monitored by ultrasound every 3–5 days, and the dose of estradiol was adjusted according to the endometrial thickness. When the endometrial thickness was greater than or equal to 7 mm and the progesterone concentration was less than 1.2 ng/mL, progesterone was given to initiate endometrial transformation. Single high-quality blastocyst transfer was performed on day 5 after endometrial transformation. Luteal phase support after transfer was consistent with endometrial transformation. Pregnancy was determined by β-human chorionic gonadotropin (β-hCG) testing on days 10–12 after FET. The first B-ultrasound was performed 14–16 days after β-hCG testing, and the second B-ultrasound was performed 14–16 days after the first B-ultrasound (Fig. 2)