Abstract
BACKGROUND: Endometrial thickness independently predicts pregnancy outcomes in frozen embryo transfer (FET) cycles. Thin endometrium always results in implantation failure and worse obstetric-neonatal outcomes. However, it has not been reported which endometrial preparation strategy achieved optimal outcomes in patients with thin endometrium undergoing FET cycles. METHODS: This historical cohort study was conducted on 2671 women with thin endometrium who underwent their first FET cycle at the Reproductive Medicine Center of a university-affiliated hospital between January 2018 and August 2022 (followed up to August 2023). Patients were divided into three groups according to endometrial preparation protocols (NC: natural cycle, AC: artificial cycle, GnRH-a + AC: AC with gonadotropin-releasing hormone agonist pretreatment). Thin endometrium was defined as endometrial thickness < 8 mm on the first day of progesterone administration. Patients with uterine abnormalities, recurrent spontaneous abortion, or donor oocytes were excluded. We also further analyzed the condition of endometrial thickness < 7 mm. Pregnancy and obstetric-neonatal outcomes were assessed. RESULTS: A total of 2671 patients were included in the study. Among patients with endometrial thickness < 8 mm, the clinical pregnancy rate was 36.2% (691/1908) in the AC group, 35.2% (178/506) in the GnRH-a + AC group, and 33.9% (87/257) in the NC group. The live birth rates were 26.8% (512/1908), 25.3% (128/506), and 27.6% (71/257) in the three groups, respectively. No statistical differences were observed in pregnancy rates or obstetric-neonatal outcomes in pairwise comparisons, except that the biochemical pregnancy loss rate in the NC group was significantly lower than that in the AC group (3.9% versus 8.6%, P < 0.05). Furthermore, this result remained consistent after multivariate logistic regression (crude odds ratio [95% CI]: 0.428 [0.223,0.821], adjusted odds ratio [95% CI]: 0.444 [0.230,0.856]). For patients with endometrial thickness < 7 mm, there were no significant differences in any outcomes across the three groups. CONCLUSIONS: Analysis using the 8 mm cut-off revealed a lower biochemical pregnancy loss rate in the NC group compared to the AC group. In contrast, no significant differences were observed in clinical pregnancy, live birth, or obstetric-neonatal outcomes based on endometrial preparation strategy for patients with an endometrial thickness < 7 mm or 8 mm. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s12978-025-02166-z.