The Duchess of Cambridge faces a third day in hospital as she is treated for a severe form of morning sickness.
Kate was "continuing to feel better", St James's Palace said on Tuesday night.
A spokesman said: "She and the Duke are immensely grateful for the good wishes they have received.
"She will remain in hospital at present and will continue to be treated for hyperemesis gravidarum."
Relief of his pregnant wife's improving condition was evident in William's smile as he emerged from the hospital after spending the day at her bedside.
The Duchess was admitted to London's Edward VII hospital on Monday, shortly before news of her pregnancy was revealed.
She has developed the condition hyperemesis gravidarum, which can cause severe vomiting and leave patients feeling tired, dizzy and suffering from headaches, caused by dehydration.
The Queen's former gynaecologist Marcus Setchell, who delivered the Countess of Wessex's two children, is treating her, according to reports.