Clomid vs Letrozole

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What’s the difference between the two? I have tried Letrozole 2 times. Once with <a href="https://glowing.com/glow-fertility-program">IUI</a> and the second time just with timed intercourse and was unsuccessful. We changed fertility clinics and are now moving onto 3 cycles of Clomid. Has anyone tried both? What was your experience? We have unexplained infertility and have been TTC for 2.5 years. Praying for a 2021 baby 🤞🏻🤞🏻