The couple have reportedly planned a low-key celebration of the special day at their Kensington Palace residence.
Kate, who is now believed to be around five days overdue, was seen in a relaxed mood driving Prince George to his regular swim at Buckingham Palace.
Swimming is said to help bring on labour, leading to speculation that the Duchess of Cambridge may have joined her son in the pool after she was seen behind the wheel of her Range Rover.
She later drove back to her Kensington Palace home, with a royal protection officer in the passenger seat and Prince George and his nanny in the back seat, to continue waiting for the arrival of her second child.
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The armed officers will also patrol the corridors of the Lindo Wing once she is inside, according to former head of royalty protection Dai Davies.
"The Duchess is overdue so it's not surprising that they are there. It is similar to a royal visit, when the police will be at the venue ahead of the royal family's arrival, only in this instance they don't know when that arrival is going to be," Davies was quoted as saying by The Telegraph.
According to him, despite the emergence of Islamic State terrorists, it was the threat of a "fixated individual" obsessed with the royal family which posed the more likely danger.
The world's media and royal watchers have been swarming outside St Mary's Hospital awaiting the royal birth.