Prince William's wife, Catherine, is set to mark her 31st birthday quietly and privately.
St James's Palace said the Duchess of Cambridge would mark the occasion "privately" on Wednesday. She is understood to be with her husband at their home in north Wales.
Kate was treated last month for a severe form of pregnancy sickness known as hyperemesis gravidarum. Symptoms include sustained bouts of nausea and vomiting.
The Duchess was admitted to hospital on December 3, a development which prompted the palace to announce that she was in the very early stages of pregnancy.
Kate made her first public appearance since falling ill when she presented cyclist Bradley Wiggins with the main award at the prestigious BBC Sports Personality of the Year awards ceremony in London on December 16 .
The British government is pushing through a bill which will end the principle of male primogeniture, meaning William and Kate's first child will succeed to the throne, regardless of gender.