I did 5 rounds of clomid before they had me do a trigger shot with round 6 when I did IUI. The reason I did the trigger before u/s is because when I went in on CD 15 my follicles were alread at 24mm and wouldn't release an egg so I had to do the trigger to help them along. It was a $60 shot that was not covered by insurance, had to go to a special pharmacy to get it. I would think with IVF though that they are taking the eggs out and all the trigger does is make sure they are released. It doesn't help to make the follicles bigger or more of them. Does that help?
I am not sure, but I just wanted to mention that I am here in the US and did take clomid. I did NOT have the trigger shot. I think a reason why clomid may be given without the trigger shot is because 1) it's an added expense, and 2) you might pop out an egg with clomid alone. Also, I wanted to mention that my regular ob would not have given me a trigger shot - I would have had to seen the specialist for that and my insurance would NOT have covered the shot.
I hope that helps to clear up some stuff, but it may not have! Sorry. :)