Study population
This was a retrospective study of patients who received IVE-ET treatment from June 2018 to December 2019 at our reproductive medicine center and the study was conducted in accordance with ethical standards of China Medical University (2021PS014F). We included patients who (1) were undergoing their first FET cycle due to failure in their fresh ET attempt or adopting a freeze-all strategy in their fresh cycle, (2) were aged 20-45 years, (3) one or two embryos of good quality or higher for transfer (embryos were evaluated for cleavage on day 3 based on the Istanbul consensus 11, and blastocyst morphology was evaluated according to the Gardner criteria 12), (4) underwent endometrial preparation with hormone replacement therapy (HRT) in the FET cycles, (5) uterine length 70-80 mm and uterine anteroposterior diameter 20-30mm, (6) the endometrial thickness on ET day was more than 8mm, and (7) had a uterus in the “anteverted and anteflexed”, “retroverted and retroflexed”, and “horizontal” positions.
The exclusion criteria were as follows: (1) uterine abnormalities, (2) endometriosis or adenomyosis, (3) endometrial polyps or uterine myomas, (4) autoimmune, endocrine, or metabolic diseases, (5) previous diagnosis of uterine adhesion, (6) history of recurrent miscarriage, (7) chromosomal abnormalities, (8) undergoing blastocyst biopsy for pre-implantation genetic testing, (9) the endometrial thickness were less than 8mm on ET day, (10) uterus in the “anteverted and retroflexed” or “retroverted and anteflexed” position, (11) missing data, and (12) ectopic pregnancy as pregnancy outcome. Total number of patients before inclusion criteria were 1703. According to inclusion and exclusion criteria, 526 patients who underwent their first FET cycle were enrolled (Figure S1). Among them, 35 patients failed in their fresh ET attempts and 491 patients adopted a freeze-all strategy in their fresh IVF cycles. The overall demographic data, IVF characteristics, and pregnancy outcomes of the whole study population are detailed in Table 1.