The spokeswoman for Pregnancy Sickness Support welcomed William's decision to cancel a public appearance in order to support Kate.
She said the the Duchess's condition is so fraught that the journey home from hospital to Kensington Palace could have triggered more bouts of nausea and sickness.
Even a car journey could have set off that downward spiral and by the time you get home you're on the way to being dehydrated.
Having support from her partner makes the whole thing more bearable.
The Duchess of Cambridge left hospital today, after three days of treatment for a severe form of morning sickness.
A nurse disclosed information about Kate's condition to two DJs who rang the hospital claiming to be the Queen and Prince Charles.
The Duchess of Cambridge is to spend another night in hospital being treated for acute morning sickness but is said to be feeling better.