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.