That’s a really good question and one that I don’t have a finite answer to. Since HG has always seemed to exist, we know it’s not a new thing, but the research on it is and as of yet all we seem to know for certain at this point is that two genes are likely the culprit for “severe morning sickness” in pregnant women. If this is the case, I’m not sure why HG would be present in a second pregnancy but not the one before it.
So here’s a few of my theories (and only mine, I’m not speaking as an expert or a researcher, just as someone who has it):
You had HG with the first pregnancy but it wasn’t as “bad” as this time around. I think this is unlikely, since HG is hard to ignore. But we do forget the bad parts of pregnancy and labor a bit, right? It’s possible you’re glorifying your last pregnancy as “not being that bad” when in reality, if you were vomiting and losing weight beyond the norm for morning sickness, you were probably dealing with HG and since the condition seems to worsen for most women with each pregnancy, this time around is worse than before.
Your pregnancy ended early. If you had a miscarriage before 6-8 weeks, it’s possible that your pregnancy didn’t progress far enough along to experience HG symptoms which typically appear after six weeks.
There’s something about those two genes that probably cause hyperemesis gravidarum that we just don’t understand yet! Perhaps they don’t always “wake up” until some of the other things happen that didn’t happen in your first pregnancy.
You don’t have hyperemesis gravidarum. Hold up - I HATED being told my pregnancies were normal so I know how crappy it is of me to say that. But something I’ve noticed in my friends who have had different gendered pregnancies is that my friends who started off pregnant with boys did just fine with only slight discomfort through their first trimester, but when they carried a girl in their next pregnancy, they felt really awful in comparison. She was usually vomiting in the morning or at least having a hard time getting through breakfast and feeling generally run down, worn out, queasy and sick through their first 14 weeks. I’ve also had friends who carried their girly pregnancies fine but felt really awful with the boys. So, I think that sometimes our bodies react poorly to either the testosterone or estrogen and if your first pregnancy was a different gender than this pregnancy, maybe that’s what’s going on. Again, NOT an expert and my opinion only. No matter what, if you have a hard time keeping up with your daily responsibilities and keeping food down you need to talk to your OB about how to feel better and take good care of yourself through your pregnancy.