Hyperemesis gravidus is more serious than "morning sickness". It is called "hyper" (high)
emesis (vomiting) because the woman usually vomits every time she tries to eat. In my case, when I had it millions of years ago, I would vomit even an aspirin and certainly vomit every pill or capsule. I would retch at the mere smell of coffee. I was hospitalized on IV twice, and was perfectly well while on the IV, but in my case there was a secret reason for this. I had the "royal disease" porphyria, in which glucose IV's are the basic treatment for an attack of porphyria. So I was not sick while on IV's. I could eat the high calorie diet they brought me (Napoleons and steak) without vomiting at all. I took no liquid by mouth. But the minute I went home, off the IV, I would vomit. So my husband was disgusted, and I left him four months later after a lot of verbal abuse about my "mental illness". My child is 50 years old now. He was a large baby, not sickly at all. But I lost a lot of weight those first three months, then gained it back and more!
Catherine probably does not have hyperemisus gravidus because of Porphyria. It could be because she's too thin anyway, but not too likely. There is always the very faint possibility that she is sick because the child has porphyria, which is known to make miserable some of the royals, but not too likely, and I would not worry about it. I hope she becomes hale and hearty as I did after three and a half months. But it made everyone in my husband's family angry that I was such a wimp. No doubt Kate will be coddled and all will go swimmingly.