Effect of follicle size on pregnancy outcomes in patients undergoing first letrozole-intrauterine insemination

Abstract Background 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. Methods 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. Results 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 E 2 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. Conclusions 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.