Everything I’ve read about William and Kate says they are keen to give George a brother or sister at some point. I don’t know much about hyperemesis gravidarum, but in Kate’s case it did not threaten her life and I believe she was fine once she got into her second trimester. I think she’s willing to take the risk.
Her life is a sunny spot under magnifying glass every day of the week. If she can raise a family in that, good for her.
Personally, anything past living her entire life and not throwing open a window on occasion to scream at the lens-jackals to “F-ck Off!” seems like gravy.
I would personally be unwilling to take that kind of risk. Anyway, it’s a decision only they can make, and it’s always possible that another disease state was a factor.
My SIL met several of the criteria for HG during her first pregnancy but was never officially diagnosed because she never became dehydrated. Right after the baby was born, she was discovered to have gallstones, which run strongly in her family, and had her gallbladder removed. Her second pregnancy was MUCH easier, and we all think this may be why.