I heard back from Dr Candour. Why he can't train the people in his clinic to speak like this, I don't know. Here is his email, verbatim, as I thought it was clear, to the point, and told me something I didn't already know:
A day 21 start really means a mid-luteal start ie in between ovulation and a period. Our nurses are advised to keep it as day 21 as this will cover the vast majority of patients. Strictly speaking the doctor's advice was more 'accurate' although with the average length of suppression being around 14 days my honest opinion is that it would not make a significant difference. Time issues with IVF only become critical around the time that hCG is given.
My advice would be to start as midway between ovulation and your 'expected period' as you can best estimate. Should we get this wrong and you begin on the cusp of a period then this is not problematic, about 30% of my patients have an 'early follicular or' day 2 start. The pregnancy rates are identical, a day 21 start is associated with a lower incidence of cyst formation, which is the reason we are currently favoring it.
I hope this answers your questions. I'm in XX at present giving a talk, but will be back on Sunday night.
I think I'm quite lucky to have found Dr Candour.
I think I will start on day 19, just not to tempt the fates too much. That's Tuesday.
I have a question for you all. How long did you stim for? I'm trying to do some planning (yes, so the powers that be can laugh at me) for this and don't know if I should be estimating 10 days or 14 days or somewhere in between. My clinic says 14 days maximum, but I have no idea how much less than that is possible. Any help much appreciated as always.