Kate and William, both 36, are hoping to give Princess Charlotte a little sister.
'They'd love to have two boys and two girls,' the source tells Life & Style, noting that Kate is particularly close to her younger sister Pippa Middleton, 34, while William considers his brother, Prince Harry, 33, his best friend.
'It's important to Kate for her own children to have a similar bond with their siblings, and she knows it's more likely to happen if they're close in age.'
The royal couple are already proud parents to Prince George, 4, Princess Charlotte, 3, and Prince Louis, 3-weeks.
The Duchess of Cambridge is known to have suffered from severe morning sickness, called hyperemesis gravidarum, with each of her pregnancies.
Kate has previously spoken about what she calls the 'huge challenge' of motherhood.
'Nothing can really prepare you for the sheer, overwhelming experience of what it means to become a mother,' she said. 'It's full of complex emotions of joy, exhaustion, love and worry all mixed into one.
'It's right to talk about motherhood has a wonderful thing. But we also need to talk about its stresses and strains. It's OK not to find it easy, and asking for help should not be seen as a sign of weakness.'
Kensington Palace is yet to respond to Life & Style's report, but it's clear Kate is a doting mum - with the Duchess spotted giving daughter Charlotte a loving kiss on the head when the family attended the Maserati Royal Charity Polo Trophy in Gloucester, England, on June 10.
The Duchess wore a simple blue dress for the equestrian event.
The off-the-shoulder design by fashion store Zara, retails for around $70.
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