because she's using Lupron as well. So I really can't speak to her scenario. When is she being instructed to take the HCG?
But want to make sure that we are clear - there's HCG trigger and HCG booster. Both use HCG (whetehr it's Novarel, Pregnyl, Ovidrel, whatever HCG). Triggers are used to release the egg and boosters are used in the luteal phase to help women with poor luteal function. A woman can be "prescribed" only a HCG trigger, only an HCG booster or both an HCG trigger and an HCG booster.
So how do "THEY" know when to give you a booster? It's basically a decision based on learnings of your luteal function from prior luteal phases. Using regular P4 support should be enough but for some women it's not.