Her husband, Prince William, arrived Tuesday morning to visit her at King Edward VII Hospital, after spending much of the day at her bedside Monday.
The news of the expected baby was announced in the palace after being admitted on Monday.
The duchess is likely to remain in hospital for several days, the palace said Monday.
The palace said her illness is Hyperemesis Gravidarum, which involves nausea and vomiting more severe than typical morning sickness.
In a statement sent by the palace said “As the pregnancy is in its very early stages, Her Royal Highness is expected to stay in hospital for several days and will require a period of rest thereafter,”
Catherine, 30, is not yet 12 weeks pregnant, Clarence House told CNN, so the palace is not announcing a due date for the child.
William and Catherine’s child will be next in line to the British throne after William, regardless of whether it is a boy or a girl, the British Cabinet Office said Monday. Planned changes to the law of succession that end the tradition of a boy jumping over an elder sister are already in effect, the Cabinet Office said.
William and Catherine were married at Westminster Abbey on April 29, 2011, in London’s biggest royal wedding in three decades and will be expecting their first child after 19months of marriage.
The baby would be the first grandchild of the heir to the British throne, Prince Charles, and the late Diana, Princess of Wales.