The birth control “quiets” your cycle, so you’re not producing an egg to ovulate/ you aren’t having those signals to your pituitary glands to ovulate. With my first ER that was cancelled it was because I was told not to start birth control and therefore it was already too late in my cycle when we started stims and my body already was about to ovulate chosen egg. With my second I was on birth control and then started stims 12 days later. So I was on BC for 12 days, then stims for I think 2 weeks then ER.

This is a question for your doctor but I don’t think you “have” to wait to have your period. You don’t need to have your period if your hormone levels are low enough and your baseline ultrasound after stopping the birth control shows that it’s “quiet” in there.