What is the optimal follicular size before triggering ovulation in intrauterine insemination cycles with clomiphene citrate or letrozole? An analysis of 988 cycles

Objective To determine the optimal size of the leading follicle before human chorionic gonadotropin (hCG) administration in cycles with clomiphene citrate (CC) and letrozole, and to examine any differences in the optimal leading follicle size between cycles with CC and letrozole. Design A retrospective study. Setting University hospital-based reproductive center. Patient(s) 1,075 women undergoing intrauterine insemination cycles with CC or letrozole. Intervention(s) Leading follicle diameters and endometrial thickness were recorded 24 hours before hCG administration, together with other cycles parameters, and were compared between pregnant and nonpregnant patients. Main Outcome Measure(s) Leading follicle diameter and intrauterine insemination outcome. Result(s) Eight percent of patients (n = 87) were excluded because their leading follicle was less than 18 mm by days 11 to 13. Pregnancy was recorded as clinical pregnancy with fetal heart activity seen at 6- to 7-week transvaginal ultrasound. For both CC and letrozole, higher pregnancy rates were achieved when the leading follicles were in the 23 to 28 mm range. The optimal size of the leading follicle was not statistically significantly different between cycles using CC or letrozole. However, for each endometrial thickness, the optimal follicular size of the leading follicle was different. Each additional millimeter of endometrial thickness increased the optimal follicular size by 0.5 mm. Thicker endometrial lining led to a higher probability of pregnancy. Conclusion(s) The optimal size of the leading follicle in ovulation induction with CC and letrozole is similar for both drugs and is closely related to the endometrial thickness.