The card was sent in 1937, when Frank was eight, and was addressed to one of her best friends, Samme Ledermann.

The Anne Frank museum has authenticated the card, which shows a clover-covered bell above a snowy field, and wishes "good luck for the New Year".

Frank, who wrote her diary while in hiding from the Nazis, died in Belsen concentration camp in 1945.

Paul van den Heuvel, a school teacher, was looking through items in his father's antique shop in Naarden, near Amsterdam, when he came across the card.

"I just found it in a box, which probably came from an Amsterdam flea market," he told Dutch television.

Their mission is to tell the truth about motherhood in their cards, to capture the craziness, messiness and hard work that makes the job a joy.

"There was a lot of pastel and fluffiness," said Hauber of Colorado Springs, Colo. "There wasn't much humor, much reality, much color and vibrancy."

Hauber is a graphic designer with two kids -- Eli, 3, and Emerson, 10 months. Chmel is a Denver-based freelance writer who also has two kids.

Chmel wanted to start Motherhood with Attitude and needed an artist. Hauber was toying with what she calls the grittiness and messiness of motherhood in her art, and needed the proper medium. When the two freelancers met on a project, the match was made.