Catherine, Duchess of Cambridge reportedly had to persuade her husband Prince William to have another baby.
The 35-year-old royal and the Prince already have Prince George, four, and Princess Charlotte, two, but recently announced they are expecting another baby, after the Duchess convinced the Prince of her desire to have another child.
A source said: "As one of three, Kate wanted at least three. William was more reluctant, not just because he was one of two, but because George was quite a difficult baby."
However, Princess Charlotte has apparently proven to be much easier for the royals to handle and Prince William has subsequently grown to support the Duchess' plans to expand their family.
The insider told the Daily Express newspaper: "Then they had Charlotte and she was a lot easier. Now the idea of a newborn doesn't seem half as challenging, they've done it twice before and they've got all their staff in place."
Earlier this month, meanwhile, the Duchess of Cambridge's uncle said he thinks she and Prince William will have more than three children.
Gary Goldsmith was overjoyed to hear that his niece is set to become a mother for the third time and has predicted they will continue to add to their family.
The wealthy business owner said at the time: "It's the best news I've had so far this year.
"She's a natural mother.
"I wonder if she will stop at three? I doubt it. They are having such fun with the babies."