Mine goes wrong every time. I won't elaborate on the last one
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My best tips are: paracetamol + nurofen plus makes you bombproof, and if it is the wrong time of the month, you can change the appointment to a better time.
Also, I have never been given a 'gown'. The first time I had to make do with my coat so I always take along a comfortable cardigan & book to keep me warm & take my mind off it whilst waiting half undressed.
Since Covid, it seems to be straight in to the room for mamo, then dress in a cubicle. Unfortunately you have to wait outside the portakabin beforehand (and I was really embarrassed by the long queue that formed after they had to shut down and recallibrate the machine. It was January)