The couple joined the Duchess' parents Michael and Carole Middleton and her siblings James and Pippa in the capital on Wednesday evening to see the company's new show Kooza. They enjoyed the spectacle from the royal box, though St James' Palace stressed the family outing was entirely "private".
The birthday treat marked the first time the Duchess of Cambridge has been seen out in public this year. Though the birthday girl was spotted at church with William on Christmas Day near her family home in Berkshire. St James' Palace asked that no pictures of the couple were published on the internet and appeared in foreign publications.
Royal followers are waiting with excitement for the announcement of the month in which the couple's first child will be born. Royal aides have said this would be made known shortly after Kate had her 12 week scan.
It's at this point expectant mothers usually announce their pregnancies, though Kate and William made the decision to make her pregnancy public knowledge after she was hospitalised with hyperemesis gravidarum.
Whether or not the sonographer makes a guess at the baby's gender at this stage, or if the couple decide they want to keep this as a surprise remains to be seen. Either way, whether Kate is carrying a boy or a girl, it will not become public knowledge until the baby is born in the summer.
One thing is sure though, the little boy or girl will be the future monarch, as a new bill which will end the principle of male primogeniture will be in effect by the time the baby is born. If they have a daughter, she is ensured the title 'Princess', after the Queen issued a decree confirming the royal title.
Under a century-old decree by George V, only the eldest son of the Prince of Wales's eldest son could have the title prince. So any daughter born to William and Kate would have been called "lady."
But the new degree, which was decided by the monarch on New Year's Eve states: "The Queen has been pleased by Letters Patentunder the Great Seal of the Realm dated 31 December 2012 to declare that all the children of the eldest son of the Prince of Wales should have and enjoy the style, title and attribute of royal highness with the titular dignity of Prince or Princess prefixed to their Christian names or with such other titles of honour."