William has already proved that he is a tower of strength for Kate. During the early stages of her pregnancy, he drove his wife to Edward VII hospital in London when her morning sickness became so severe that she was dehydrated.
Prince William has been by his wife's side every step of the way
"I don't know why they call it morning sickness," he said afterwards. "They should call it all day and all night sickness. She feels like it is going to go on forever."
Royal husbands have not always been so present during their baby's birth. When the Queen – then Princess Elizabeth was having Prince Charles in 1948 – her husband Prince Philip was nowhere near the temporary labour ward that had been set up on the first floor of Buckingham Palace in a room overlooking the Mall.
Instead he was playing squash with his friend and private secretary Michael Parker. He is said to have toasted the health of the little Prince with champagne and later took his wife some carnations.
In Prince Philip's day husbands were banished from the birthing suite
His son broke royal protocol to be at the births of both his boys in the mid-Eighties. Shortly before announcing they were expecting their first child Diana and Charles visited a maternity ward in Wales.
A newspaper report said: “The princess had asked about labour and the Prince said he thought it a good thing for fathers to be present at the birth.”
His support was crucial because the Princess later told her biographer Andrew Morton that she had to be induced because the pressure when she was expecting William was so intense. "It was becoming unbearable. It was as if everybody was monitoring every day for me," said Diana, who was aged 20 at the time.
Prince Charles was the first royal father to be present during labour
At the time of Prince Harry's birth the Prince and Princess of Wales were closer than they had ever been: it was noted that they held hands in the back seat of a blue Ford Granada on their way to hospital.
Times have changed so much it is unthinkable that William would want to be anywhere than right there with Kate as she brings their longed for first-born into the world.
Watch all the action from outside the Lindo Wing as the world awaits news of the arrival of the Prince or Princess