She said she was "feeling better" when she checked out of hospital last week, but the Duchess of Cambridge was bedridden again yesterday when her acute morning sickness returned.
Kate fell ill on Sunday morning, forcing Prince William to cancel an engagement to stay by her side.
The Duchess didn't require further hospitalisation and is recuperating in her Kensington Palace home.
"It is well known that hyperemesis gravidarum often recurs," a St. James Palace spokesman said.
William had been due to attend the British Military Tournament in Earl's Court, but pulled out of the event to wait on Kate.
The previous evening, he donned a tuxedo to attend a black tie gala at London's Royal Albert Hall.
He was in great spirits at the event, joking with onlookers about his wife's illness.
"I don't know why they call it morning sickness they should call it all day and all night sickness," William said.
"[Recovery is] a long old process but she is getting there. She feels like it is going to go on forever."
Kate was forced to announce her pregnancy last Monday when she was hospitalised for hyperemesis gravidarum, a rare complication of pregnancy that affects less than two percent of pregnant women.
It causes constant nausea and vomiting, making it nearly impossible to eat or drink. It can last throughout the pregnancy, and even continue for several weeks after the child is born, meaning Kate is unlikely to carry out any more engagements.
We won't be hearing much more about her condition however, as the Palace has confirmed it won't be providing weekly updates.
"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," a statement read.