The Duchess of Cambridge will not be attending the opening ceremony and start of the Invictus Games, due to her severe morning sickness.
Earlier this week, Clarence House confirmed the Duchess of Cambridge is pregnant.
A statement read: "Their Royal Highnesses The Duke and Duchess of Cambridge are very pleased to announce that The Duchess of Cambridge is expecting their second child."
"As with her first pregnancy, The Duchess is suffering from Hyperemesis Gravidarum [acute morning sickness].
"The Duchess of Cambridge is being treated by doctors at Kensington Palace."
The Invictus Games has been organised by Kate Middleton's brother-in-law Prince Harry. Designed for wounded soldiers, more than 400 competitors from 13 countries will take part.
Kensington Palace confirmed: "The Duchess of Cambridge will not attend the opening ceremony of the Invictus Games this evening or the athletics event tomorrow."
The opening ceremony for the first Games will be hosted in the Queen Elizabeth Olympic Park in London tonight (10 September). It is being put together by Kim Gavin, the creative director of the 2012 Summer Olympics Closing Ceremony.