JBRA Assisted Reproduction 2025;29(Suppl.2 SBRA 2025):204
Poster Presentation
29^th Annual Congress of the SBRA. São Paulo/SP - Brazil, 2025
doi: 10.5935/1518-0557.20263881

P-192. Reduced Miscarriage Risk Associated with Larger Follicle Size at Trigger in Modified Natural Cycles with Euploid Blastocyst Transfer

Flávia Rocha Torelli¹, Luciana Carvalho Delamuta¹, Cassia Raquel Teatin Juliato¹, Georges Fassolas¹, Joao Antonio Dias-Jr¹, Luiz Fernando Oliveira Henrique¹, Carlos Roberto Izzo¹

¹ Originare - São Paulo - SP - Brasil

Objective: To investigate the association between pre-trigger clinical and hormonal parameters and pregnancy outcomes— including live birth and miscarriage—in modified natural cycles with single euploid blastocyst transfer.
Methods: This is a secondary analysis of a cohort study that evaluated 94 women who underwent frozen embryo transfer (FET) in modified natural cycles between January 2022 and December 2023 at a private fertility center. Primary variables were late gestational outcomes (specifically live birth and miscarriage rates) regarding pre-trigger hormonal and follicular parameters. Cycles were characterized by spontaneous follicular growth followed by ovulatory triggering with subcutaneous hCG. Transvaginal ultrasound assessments were performed serially from the onset of menstruation to monitor endometrial development and dominant follicle growth. When the follicle reached ≥14 mm and the endometrial thickness was ≥6 mm, serum estradiol (E2) and progesterone (P4) levels were measured prior to triggering. Luteal phase support was administered with vaginal progesterone alone or in combination with oral formulations. Embryo transfer was performed five days after initiating supplementation. Only day 5 or 6 euploid blastocysts derived from autologous oocytes were transferred. All patients had pre-trigger serum P4 levels <1.0 ng/mL. Serum β-hCG was measured 9 to 10 days after FET, and clinical pregnancy was confirmed by transvaginal ultrasound four weeks post-transfer. Associations between clinical and hormonal variables and pregnancy outcomes were evaluated using univariate and multivariate logistic regression models with stepwise variable selection. The Youden index was applied to determine optimal thresholds for E2 levels and follicular diameter to predict live birth and miscarriage risk. A p-value <0.05 was considered statistically significant.
Results: The mean age at IVF was 36.88±2.79 years, and 38.02±2.67 years at FET. On the day of trigger, the average follicular diameter was 16.94±1.81 mm, and mean endometrial thickness was 8.25±1.29 mm, with a trilaminar pattern observed in all cases. Ovulatory trigger was performed at a mean of 12.10±2.29 days into the cycle. A history of previous childbirth was reported by 22.58% of participants. The live birth rate (LBR) was 59.57%. No significant differences in age, body mass index (BMI), luteal support regimen, or the presence of adenomyosis or endometriosis were observed between women with and without live birth. In the overall population, multivariate analysis revealed that prior childbirth was significantly associated with live birth (OR=3.71; 95% CI: 1.29–10.67; p=0.015). Serum E2 levels ≥163 ng/mL on the day of trigger were also predictive of live birth (OR=3.07; 95% CI: 1.04–9.08; p=0.048). Among women who achieved a positive β-hCG, the mean level was 258.73±304.32 mIU/mL, and significantly higher in those with subsequent live birth (p<0.001). In this subgroup, the presence of a dominant follicle measuring ≥17 mm on the day of trigger was associated with a lower risk of miscarriage, corresponding to a 6.98-fold increase in the odds of live birth (OR=6.98; 95% CI: 1.40–34.73; p=0.018).
Conclusion: In modified natural cycles with single euploid blastocyst transfer, prior childbirth and elevated serum estradiol levels on the day of trigger were positively associated with live birth. Additionally, a dominant follicular diameter ≥17 mm was associated with a lower risk of miscarriage among women who conceived. These findings underscore the predictive utility of individualized hormonal and ultrasonographic monitoring at the time of ovulatory trigger for optimizing pregnancy outcomes.