The duchess was 'glowing' as she attended the official function at Buckingham Palace, showing no signs of the 'severe and extreme' morning sickness that has seen her bedridden behind palace doors since August 30.
Despite Palace reps saying she's 'still not out of the woods' with hyperemesis gravidarum - the same condition she battled while pregnant with George and Charlotte - she put on a brave face at the reception, beside Prince William, 35, and Prince Harry, 33.
And insiders tell New Idea that her amazing red-carpet baby bump debut is all thanks to her 'secret weapon' - her new best friend Meghan Markle.
It's been revealed that the pair have grown 'extremely close' since Harry's girlfriend began living at Kensington Palace, with Kate, 35, relying heavily on the Suits actress during the tough first few months of her pregnancy.
'Meghan really has been her secret weapon in helping her through the difficult early stages of her pregnancy and illness,' the source reveals.
'They've grown very close, very quickly and Kate's been so grateful to have her by her side through these long weeks of sickness. Even when she's not there, they FaceTime and Skype a lot - they speak every day, often more.'
Adds another source: 'Meghan helped Kate choose the beautiful lace Temperley frock she wore. She really wanted to make sure she felt beautiful, despite not feeling her best.'
It comes as no surprise that 36-year-old Meghan has become Kate's rock. Friends of the actress say they have a lot in common and have really been there for one another. Kate was able to help guide Meghan through her first public appearance with Harry recently at the Invictus Games in Toronto, giving her lots of pointers and helping to ease her nerves.
'She's in bed most of the time, with no-one to talk to, except for William. So having Meghan stop by or on the other end of the phone has been a godsend.'
Meghan has even offered to help share the load of Kate's charity commitments as her pregnancy progresses and after the baby is born.
'Kate is so happy she has Meghan in her life now, it's made the world of difference.'
For the full story see this weeks issue of New Idea.