Letrozole has been proven to be an effective method for inducing ovulation. However, little attention has been paid to whether the lead follicle size will affect the success rate of intrauterine insemination (IUI) with ovulation induction with alone letrozole. Therefore, we hope to investigate the effect of dominant follicle size on pregnancy outcomes on human chorionic gonadotropin (hCG) day of the first letrozole-IUI. A retrospective cohort study design was employed. We included patients with anovulation or unexplained infertility undergoing first IUI treatment with letrozole for ovarian stimulation. According to the dominant follicle size measured on the day of hCG trigger, patients were divided into six groups (≤ 18 mm, 18.1–19.0 mm, 19.1–20.0 mm, 20.1–21.0 mm, 21.1–22.0 mm, > 22 mm). Logistic models were used for estimating the odds ratios (ORs) with their 95% confidence interval (CIs) for achieving a clinical pregnancy or a live birth. A restricted cubic spline was drawn to explore the nonlinear relationship between follicle size and IUI outcomes. A total of 763 patients underwent first letrozole-IUI cycles in our study. Fisher exact test showed significant differences among the six follicle-size groups in the rates of pregnancy, clinical pregnancy and live birth (P < 0.05 in each group). After adjusting the potential confounding factors, compared with the follicles ≤ 18 mm in diameter group, 19.1–20.0 mm, 20.1–21.0 mm groups were 2.3 or 2.56 times more likely to get live birth [adjusted OR = 2.34, 95%CI (1.25–4.39); adjusted OR = 2.56, 95% CI (1.30–5.06)]. A restricted cubic spline showed an inverted U-shaped relationship between the size of dominant follicles and pregnancy rate, clinical pregnancy rate, and live birth rate, and the optimal follicle size range on the day of hCG trigger was 19.1–21.0 mm. When the E2 level on the day of hCG trigger was low than 200 pg/mL, the clinical pregnancy rates of 19.1–20.0 mm, 20.1–21.0 mm groups were still the highest. The optimal dominant follicle size was between 19.1 and 21.0 mm in hCG-triggered letrozole-IUI cycles. Either too large or too small follicles may lead to a decrease in pregnancy rate. Using follicle size as a predicator of pregnancy outcomes is more meaningful when estrogen on the day of hCG trigger is less than 200 pg/ml.

中文翻译：

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卵泡大小对首次来曲唑宫腔内人工授精患者妊娠结局的影响

来曲唑已被证明是诱导排卵的有效方法。然而，卵泡大小是否会影响单用来曲唑促排卵宫腔内人工授精（IUI）的成功率却很少受到关注。因此，我们希望在首次来曲唑 IUI 的人绒毛膜促性腺激素 (hCG) 日研究优势卵泡大小对妊娠结局的影响。采用回顾性队列研究设计。我们纳入了无排卵或不明原因不孕的患者，他们接受了首次 IUI 治疗，使用来曲唑刺激卵巢。根据hCG触发当天测量的优势卵泡大小，将患者分为六组（≤18mm、18.1-19.0mm、19.1-20.0mm、20.1-21.0mm、21.1-22.0mm、>22mm）。Logistic 模型用于估计实现临床妊娠或活产的比值比 (OR) 及其 95% 置信区间 (CI)。绘制受限三次样条来探索卵泡大小与 IUI 结果之间的非线性关系。在我们的研究中，共有 763 名患者接受了第一个来曲唑-IUI 周期。Fisher精确检验显示，6个卵泡大小组的妊娠率、临床妊娠率和活产率差异有统计学意义（各组P < 0.05）。调整潜在的混杂因素后，与卵泡直径≤18毫米组相比，19.1-20.0毫米、20.1-21.0毫米组活产的可能性是卵泡直径≤18毫米组的2.3或2.56倍[调整后OR = 2.34，95%CI（1.25） –4.39）；调整后 OR = 2.56，95% CI (1.30–5.06)]。限制三次样条显示优势卵泡大小与妊娠率、临床妊娠率和活产率之间呈倒U型关系，hCG触发当天的最佳卵泡大小范围为19.1-21.0 mm。当hCG触发日E2水平低于200pg/mL时，19.1~20.0mm、20.1~21.0mm组临床妊娠率仍最高。在 hCG 触发的来曲唑-IUI 周期中，最佳优势卵泡大小为 19.1 至 21.0 毫米。卵泡太大或太小都可能导致怀孕率下降。当 hCG 触发当天的雌激素低于 200 pg/ml 时，使用卵泡大小作为妊娠结局的预测指标更有意义。