What do we make of that painful, emaciating first phase of the pregnancy?

As Dr. Sapirstein notes, it's expected that Rosemary will be weak, lose weight, and be in chronic pain for a while at the start of her pregnancy. Then, a few agonizing months in, the baby starts kicking and suddenly the pain stops, and Rosemary gradually (but fairly quickly) returns to a more healthy state.

What do we think is going on there in Rosemary's womb during all this? I suppose the easiest answer is that the demonic embryo is overzealously feeding on her body in an unnatural way, and the reason the pain and all that stops is because at that point the fetus has now properly formed and no longer needs to subsist on her lifeform so much.

I'm curious if anybody had any other impressions of this while watching. It's interesting how it feels like there's a specific process happening but is never outright explicated.