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Well, I can tell you this: I just had a cancelled IVF cycle (not enough eggs for IVF) that was converted to an IUI cycle. I had 6 eggs at the time of trigger but the RE only thinks 3-4 would release. Anytime you have more than one egg release you have the possiblity of multiples. Most IF meds will have you produce more than one egg because that's their purpose so the chance of multiples is always there. The only way to control the number of eggs would be to use IVF and only transfer one to two eggs.
I got pg from the IUI cycle and my betas are quite high so we don't know yet if there is more than one in there.