Intrauterine insemination (IUI) combined with ovulation induction (OI) is a commonly employed fertility treatment, with letrozole (LE), a third-generation aromatase inhibitor, frequently used for stimulating ovulation. Despite its widespread use, there is limited and inconclusive research on the ideal dominant follicle size for triggering ovulation in LE-IUI cycles. This study aimed to establish the optimal dominant follicle size at the trigger point for patients with ovulatory dysfunction (OD) and unexplained infertility (UI) undergoing LE-IUI.
Conducted as a retrospective analysis, this study involved women under 40 years of age diagnosed with OD or UI who underwent LE-IUI cycles at a reproductive medicine center in Guangzhou, China. The participants were propensity-matched on baseline characteristics, resulting in 411 cycles for each group (OD: average age 31.02 years, SD 3.44; UI: average age 31.20 years, SD 3.43).
The study revealed notable differences between the OD and UI groups. Specifically, the OD group showed significantly higher rates of positive human chorionic gonadotropin (HCG) (22.4% vs. 9.5%), clinical pregnancy (21.5% vs. 7.9%), and live birth (19% vs. 7.1%) when compared to the UI group (P < 0.001 for all comparisons).
In the OD group, patients with dominant follicle sizes of 17–18.9 mm had lower success rates in terms of HCG positivity, clinical pregnancy, and live birth compared to those with follicle sizes of 19–21.0 mm and > 21.0 mm (HCG positive: 7.6% vs. 21.5% vs. 26.2%; clinical pregnancy: 6.1% vs. 21.5% vs. 25.6%; live birth: 4.5% vs. 19.2% vs. 23.2%; P < 0.05 for all).
Conversely, in the UI group, the highest rates of HCG positivity were observed in those with dominant follicle sizes between 19–21.0 mm, while those with follicle sizes > 21.0 mm exhibited lower rates (HCG positive: 13.3% vs. 11.8% vs. 3.4%, P = 0.023).
The findings of this study suggest that for patients with ovulatory dysfunction, a dominant follicle size of ≥ 19.0 mm is optimal for triggering ovulation. Meanwhile, in unexplained infertility cases, a follicle size ≤ 21 mm may be associated with improved HCG positive rates. These results emphasize the necessity of considering the underlying cause of infertility when determining the timing for ovulation trigger. Further randomized controlled trials are needed to confirm these findings and refine clinical guidelines.
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