Kate, 31, has the blessing of the Queen for the event with family and friends after weeks of planning by her 29-year-old sis.
A friend said: “It isn’t royal tradition but Kate isn’t a traditional Royal.”
The celebrations will be a real family affair, including a cake in the shape of a dummy made by brother James, who runs his own baking firm.
Goody bags are believed to include candles and gifts from stores Jo Malone and The White Company. The friend added: “She is modern and wants a normal life.
“Prince Harry, Kate and William are the Royal Family’s link to the younger generation.
“A baby shower wouldn’t normally happen if you’re in the Royal Family, but there is now a modern way of thinking.
“It will be a great day for everyone and Kate is doing this with the blessing of the Queen.”
The date is a closely guarded secret, but it is believed it will be held at Kate’s family home in Berkshire.
Bookies Ladbrokes have released images of how it thinks the event might look to launch its royal baby odds.
The Duchess, due to give birth in July, was unable to attend a charity polo event with William in Chester on Wednesday.