The 30-year-old's condition was not serious enough for her doctors to consider sending her back to hospital, but women with the condition can experience symptoms for months and need repeated hospital admissions.
The Duchess, who spent three nights in hospital last week being treated for hyperemesis gravidarum, took a turn for the worse as she spent the weekend at Kensington Palace.
Prince William pulled out of an official engagement at the British Military Tournament at Earl's Court, London, on Sunday night, so he could look after his wife.
The news came amid a row over the apparent suicide of an Indian-origin nurse between the hospital that treated the Duchess and an Australian radio station that made a hoax call of which the nurse was a victim.
The royal had said that she was feeling "much better" when she was discharged from King Edward VII hospital on Thursday morning, and on Saturday night was well enough for the Duke to feel able to attend an engagement at the Royal Albert Hall.
But at lunchtime on Sunday, St James's Palace announced that the Duke would be "spending Sunday privately with the Duchess" instead of going to Earl's Court.
"It is well known that hyperemesis gravidarum often recurs and, until further notice, to allow the Duchess a degree of privacy during her pregnancy, we do not intend to offer regular condition checks or advise of routine developments associated with it," the Telegraph quoted a St James's Palace spokesman as saying.
The Duke is expected to stay by his wife's side until Thursday as he had taken time off from his work as an RAF search and rescue pilot to carry out several engagements, the last of which is at the royal film performance of 'The Hobbit' on Wednesday. The Duke is thought to have also been allowed compassionate leave.