I am new here (figured that if I was going to join a DDC anywhere, it would be at Mothering!). Just wanted to weigh in on this one.
Nausea and vomiting are very closely tied to HCG levels in the blood, and HCG levels start to plateau and diminish between 8 and 9 weeks (when the placenta is starting to take over hormone production, so high HCG levels are not needed anymore). Up to this 9 week mark, HCG levels are increasing nearly exponentially, which is why morning sickness tends worsen up to this point and to peak around this time. Then, HCG levels start to decrease, and this decrease is usually when women begin to feel a bit better. So, feeling a little better by 10 weeks LMP is totally normal (in fact, you could say that it's 'textbook'!), and many women say that their morning sickness just about vanishes by 12 weeks LMP.
If you look up Doc Google, on the other hand, you will find many pregnancy sites that talk about morning sickness that persists to the 14th, 16th, or even 20th week. This tends to be exception, rather than the rule. Not the right yardstick to measure against, in any case.
I understand the worry and anxiety, though. I had a missed miscarriage at 12 weeks in August (baby had stopped developing at 9 weeks), and one of the things that I noticed was that my morning sickness pretty much disappeared overnight right around that time. Disappearing, though, is much different than tapering, which it seems is what we are all experiencing. Last Friday, I had an amazing, carefree, energetic day with just a twinge of nausea (and I wasn't even anxious about it because I had heard the baby's heartbeat the day before on the nurse's doppler). Then, I had three pretty wretched days. Now, I feel a little bit better. As long as I eat regularly (ferocious appetite, anyone?), the nausea is pretty mild. But yes, at this stage, ups and downs like this are also normal. So enjoy the good days!
Hope this helps!
Hugs to all,