Great question. OHSS does get worse shortly after trigger especially with an hCG trigger (vs an agonist trigger). The symptoms peak as you rightly said 3-7 days after trigger. The benefit of freezing embryo rather than fresh transfer is the reduce the severity of the OHSS while it resolves. I typically use agonist triggers and plan to freeze if I see any risks or signs of OHSS. Even in the best of situations however OHSS can still happen. I believe the most important way to address OHSS is to have an open discussion about the signs and symptoms and make a plan with my patient about how to proceed. That being said, I do very few fresh transfers and have seen very few cases of severe OHSS since moving to that approach.