At the hospital, William, second-in-line to the British throne, visited London Air Ambulance pilots, paramedics and other staff, who presented him with a toy helicopter.
The group gave William a card for his wife's birthday, which was made by a group of children, according to the U.K.'s Hello magazine.
Prince William has spent his wife's birthday participating in training exercises at Royal London Hospital, flying with first responders in a rescue helicopter.More news: Governor removes Parkland sheriff over failure to save children
While the Duchess usually celebrates her birthday in private with her family, that didn't stop one member of the public from asking her husband William how she was spending her day. "I did remember this morning, so that was ok", he said.
Charlotte goes to Willcocks Nursery School, while George goes to Thomas's London Day School in Battersea.
Kensington Palace marked the occasion by sharing a photo of Kate with the caption: "Thank you everyone for all your lovely messages on The Duchess of Cambridge's birthday", along with a balloon emoji.
George will be celebrating his sixth birthday in July, while baby Louis, turns one on April 23.More news: How Much Liverpool Will Miss Out On If Coutinho Leaves This Month
Following the rumors about Kate's potential pregnancy, British bookies are sure there will be a royal announcement in 2019.
Kate has also become an aunt to sister Pippa Matthew's baby son Arthur, and the Cambridges are also awaiting the arrival of the Duke and Duchess of Sussex's first child in the spring - another cousin for George, Charlotte and Louis.
The 36-year-old worked as an air ambulance pilot from 2015-2017 with the East Anglian Air Ambulance.
For the second half of his day yesterday, It is expected that Prince William spent it with Kate and their children.More news: Trump says record-long govt shutdown could be resolved 'in 15 minutes'