HG is all in our heads!
No seriously though I think the chemical changes of pregnancy, primarily hormones, affect the brain. I told you about those HCG dieters. It is somewhat interesting research that Dr came across.
All brains are different. As we see, there are so many ways a body and brain can react to any given medication or substance or chemical. I don't believe it's primarily auto-immune in nature, but more on that later. I actually don't think it's a spectrum disorder. I think there's one primary cause, and I do believe it's the same cause as what is behind normal NVP. ANY bioligical process can go awry... and in our case... it's the "normal" m/s that probably originally was protective to the embryonic young.
I think HG actually is what should be m/s for us- but our brains handle the hormones differently than 99% of women. If all our brains are set to "normal" and things like viral illness or food poisoning are DESIGNED to essentially to push our brain from "normal" to very nausated (and thus vomiting to protect itself) ... the chemical changes for OUR brains sets it into 'VERY NOT NORMAL'. Again, we're all different. That would be why we all have different triggers, respond to different medications, get better at different times, etc.
Now, the placenta does communicate with our bodies. There is an auto-immune aspect to all pregnancies. However, I think that is where you may actually see a link to pre-E and HELLP and other issues that may involve the liver. I think, being nutritionally compromised (as HG leaves us too often) leaves us at increased risk (albeit probably slightly on the whole) for just about everything. I think it's certainly possible that having the two issues "auto-immune" and HG could exacerbate each other.
Many fluffy moms also end up with auto-immune issues too, so I don't think HG is just AI in nature.