Abstract
BACKGROUND: This study aims to identify the optimal dominant follicle size on the trigger day in patients with ovulatory dysfunction and unexplained infertility undergoing intrauterine insemination with letrozole (LE-IUI) cycles. METHODS: A retrospective analysis included 411 cycles of each group after 1:1 propensity score matching, comparing basic characteristics and outcomes based on dominant follicle size. RESULTS: Higher rates of HCG positive, clinical pregnancy, and live birth were found in ovulatory dysfunction versus unexplained infertility (22.4% vs. 9.5%; 21.5% vs. 7.9%; 19% vs. 7.1%, P < 0.001). In ovulatory dysfunction, dominant follicles 17-18.9 mm had lower rates of HCG positive (7.6% vs. 21.5% vs. 26.2%, P = 0.007), clinical pregnancy (6.1% vs. 21.5% vs. 25.6%, P = 0.004), and live birth (4.5% vs. 19.2% vs. 23.2%, P = 0.004) compared to 19-21.0 mm and > 21.0 mm groups. Conversely, in unexplained infertility, follicles > 21.0 mm had lower HCG positive rate(13.3% vs. 11.8% vs. 3.4%, P = 0.023) compare to other two groups. Dominant follicle size on trigger day influenced HCG positive rate in LE-IUI cycles for both groups. CONCLUSION: Triggering at follicle size ≥ 19.0 mm may be optimal for ovulatory dysfunction, while a size ≤ 21 mm may improve HCG positive rates in unexplained infertility, underscoring the need to consider infertility factors in trigger decisions. TRIAL REGISTRATION: This study is registered with China Medical Research Online (Registration Number: MR-44-23-038090S, www.medicalresearch.org.cn ).