Objective: To evaluate follicular phase parameters during ovarian stimulation with follicle-stimulating hormone (FSH) alone or with the aromatase inhibitor, letrozole.

Methods: Two groups of women undergoing IUI: group I (389 patients; mean age 35±4.3) underwent 630 IUI cycles stimulated with letrozole and FSH; and group II (134 patients; mean age 36.0±4.6 years) underwent 166 IUI cycles stimulated with FSH only. Each group was stratified into ovulatory and anovulatory cycles. Patients were monitored by ultrasound for (folliculometery) and blood sampling for (hormonal assay) on days 3, 7, 9 or 10 of the cycle, and on the day of hCG administration (d-hCG).

Results: Group I had a significantly lower follicular count >10 mm on day 7,>12 mm on day 9 or 10, and>15 mm on d-hCG compared to group II (P=0.006,< 0.001 and< 0.001) respectively. After stratifying patients by diagnosis, this relationship was maintained only for patients with ovulatory infertility (P=0.003,< 0.001 and< 0.001 respectively). Serum E2 was significantly lower in the group I ovulatory and anovulatory at the last 3 monitoring visits (P<0.001). However, the difference in E2 levels decreased in the preovulatory period with similar E2 level per mature follicle. No premature preovulatory progesterone rise was observed in either group. However, significantly lower progesterone levels were observed in the second half of the follicular phase in group I (P=0.02&<0.001). Endometrial thickness was significantly lower in group I at the 2^nd and the 3^rd visits (P<0.001, 0.01) but was comparable to group 2 at the last monitoring visit. Although, the pregnancy rates were similar between the 2 groups, the multiple pregnancy rate was significantly higher in the FSH only group (p=0.039).

Conclusion: The addition of letrozole modifies the follicular, hormonal and endometrial dynamics of FSH stimulated cycles with possible positive effects on the overall cycle outcome.