In an interview with Australia's New Idea magazine, Jessica Hay said: "Kate still looks at food and feels nauseous. She's been plagued with that feeling you have when you've had food poisoning and your stomach's shrunk.
"She finds it very hard to eat a full meal and gets full very quickly. The hypnotherapy is taking away any negative thoughts connected with food from the morning sickness, and replacing them with cravings for healthy, nutritious food."
Jessica said the Duchess, whose baby is due in July, has been eating wholesome meals and snacks consisting of food such as avacados, berries, oatmeal and broccoli.
She added: "All in all, Catherine's very happy. With so much family around her and William, they're starting to enjoy the pregnancy after such a traumatic start."
The Duchess has also taken up Pilates three times a week to keep fit during her pregnancy.