In Touch adds that both duchesses, 36, are preparing to raise the royal cousins together.
The insider spilled, 'Kate and Meghan bonded over the men they married. Now they're bonding over being pregnant together.'
Despite the two wanting to share the exciting news with the world, In Touch's bombshell report reveals the two women have only disclosed to friends and family.
On top of the exciting news, Meghan feels an 'enormous relief.'
'She was a nervous wreck about conceiving,' a palace insider says, not only because of her age but 'because it didn't happen with her first husband,' producer Trevor Engelson, from whom she split in 2013 after two years of marriage and nine years as a couple.
The pressure to give husband, Prince Harry, children 'felt like a huge weight on Meghan's shoulders,' says the source.
'Everyone expected her to get pregnant immediately.'
Meanwhile Prince William, 36, was 'initially shocked when Kate told him he's going to be a dad again', but now he sees it as a 'happy surprise.'
'He and Kate have always wanted four kids,' the source tells.
And this time around, adds the source, Kate gets to share her pregnancy journey with not only her young sister Pippa, 34, who is due with her first child in a few months, but her new sister-in-law, too.
The source adds that Kate is 'convinced she's having another girl because she's having the same cravings for sweets that she did when she was pregnant with Charlotte.'
Meghan is hoping for one of each - a boy and a girl - the insider tells In Touch.
'This is a dream come true for both couples.'
The royal family is yet to respond to In Touch's report.