A picture of Kate wearing a mask was posted on the Kensington Royal Instagram account, alongside a message thanking people for their “kind wishes” on the day.
It included a statement saying: “Birthdays have been very different in recent months, and our thoughts continue to be with all those working on the front line at this hugely challenging time.”
Kate is believed to be with her family at their Norfolk home of Anmer Hall, where the Cambridges spent Christmas after the Queen decided to have a quiet festive period at Windsor Castle.
Her birthday celebrations are likely to be a private affair, due to lockdown restrictions, with William and their three children – seven-year-old Prince George, Princess Charlotte, five, and two-year-old Prince Louis.
During the pandemic, the Cambridges adapted their public life, turning to video calls to carry out virtual engagements and when restrictions were lifted carried out events face-to-face.
In November, the duchess said she was “humbled” more than half-a-million people took part in her landmark research on the issue of early years.
The study was the largest of its kind in the UK on perceptions of early childhood – which found only one in four people recognised the key importance of the first five years of a child’s life.
But William and Kate’s trip provoked veiled criticisms from Welsh and Scottish ministers who raised suggestions about the timing of the visit while Covid cases were still prevalent.
In a lighter moment Kate, William and their children made a rare public appearance as a family when they went to a London theatre to watch a pantomime staged for key workers in the run-up to Christmas.
And the Cambridges and their children were pictured a number of times taking part in the weekly clap for carers tribute during the first national lockdown.