Prince Harry will be bumped down in the line of succession, but was overjoyed, describing the news as "fantastic," and offering a thumbs up while on a visit to Manchester.
The announcement comes as the Royal couple prepared to mark another milestone for their young family: George is scheduled to start school Thursday at Thomas's Battersea in south London.
Their choice of a south London school indicated that the royal couple was settling into their Kensington Palace apartment, having moved recently from their Norfolk home, Anmer Hall.
More recently, during a royal tour of Poland, Kate joked about a third child when she was given a cuddly toy designed to soothe tiny babies.
Katherine Redo, 34, who lives in London but is originally from Metairie, La., held tight to her squirming 2-year-old daughter, Annabelle, and said she believed many people in the United States would be happy for the royal couple.
- News from the palace: Another baby for Wills and KateThe Mercury News
- Kate Middleton's 12 most royal maternity looks of all timeINSIDER
- Pregnant Kate has acute morning sickness for 3rd timeSFGate
- Palace Announces Prince William, Duchess Catherine Expecting 3rd ChildNBC New York
- Prince William, Princess Kate expecting third childUPI.com
- Pippa Middleton Has Honeymoon Baby To Rival With Sister Kate's Latest News ...The Inquisitr
- Prince William and Kate, duchess of Cambridge, are expected a third childTampabay.com
- Britain's royal family announces third pregnancy for Duke and Duchess of CambridgeTampabay.com
- Palace announces Prince William, Kate expecting third childTribune-Review