Royal baby: A third for Kate as eldest starts school

But the announcement yesterday that the Duchess of Cambridge is expecting a third child, has a national significance.

The word came in the same week that Prince George begins lessons at Thomas’s, Battersea, and just four days after Kate and William marked the 20th anniversary of the death of his mother, Diana.

William’s brother, Prince Harry, said the news was “fantastic”, and though he is not a doctor, pronounced that Kate would overcome her morning sickness.

“I think she’s OK,” he said, as she remained inside Kensington Palace, cared for by nurses.

She has hyperemesis gravidarum, as she did in her previous pregnancies. It is a rare condition causing severe vomiting, which affects 3.5 in every 1,000 pregnant women.

Experts say the condition often gets worse with subsequent pregnancies, but those who have experienced it before can be put on medication as soon as they get a positive pregnancy test.

When expecting Prince George in 2012, Kate spent three nights in King Edward VII’s Hospital in central London before being discharged.

Meanwhile, the Prince of Wales, the new grandfather-to-be, was said to be “delighted” by the news, as were the Queen and other members of the family.

The princess is less than 12 weeks pregnant, and the palace had revealed the news early after her sickness forced her to pull out of a public engagement yesterday.

Kate had been due to visit the Hornsey Road Children’s Centre in London to hear about the mental health impact of becoming a mother.

Just weeks ago, she joked about having a third child while she toured Poland and Germany with William, George and two-year-old Charlotte.

In Warsaw, she had been handed a present designed for newborns, and had said to William: “We will just have to have more babies.”

Kensington Palace said in a statement: “Their Royal Highnesses the Duke and Duchess of Cambridge are very pleased to announce that the Duchess of Cambridge is expecting their third child.

“The Queen and members of both families are delighted with the news.”

By having more than two offspring, William and Kate are following in the footsteps of the Queen and the Duke of Edinburgh, who went on to have four children - although there was a gap of 10 years between their second child, Anne, and third, Andrew.

An age gap of less than two years between Prince George and Princess Charlotte gave Kate enough time to try for a third before she reached her late 30s.

But William - who is one of two siblings - may have needed more convincing.

On an overseas tour to Singapore in 2012, upon being asked by a group of teenagers how many children he would like to have, he said he was “thinking about having two”.

His thoughts now will likely turn to the question of whether to hire a second nanny. He and Harry had two at the same time when they were young - Barbara Barnes and her deputy Olga Powell, who later took the lead role.