Michelle Obama speaks in an undated file photo.

Elegant, passionate, a strong career woman and a devoted mom, Michelle Obama has already become a role model with an army of fans as she prepares to become the nation's first lady.

Three days after celebrating her 45th birthday, she will take her place by her husband's side on Jan 20 as Barack Obama is sworn in as the nation's first African-American president.

Obama will be one of the nation's youngest first ladies after the graceful Jackie Kennedy, who was just 31 when John F. Kennedy took office.

And while she has insisted that her main job will be "mom-in-chief" to her two daughters, Malia, 10 and Sasha, 7, her role may well evolve in the months ahead as the first family settles into the White House.

Officially Michelle Obama has said she has no political ambitions of her own, and this week resigned from her job as vice-president at the University of Chicago medical center, where she worked for 7 years.

"Even as first lady, my number one job would still be mom," she told reporters just before the Nov 4 elections. "My first priority will always be to ensure that our daughters stay grounded and healthy, with normal childhoods - including homework, dance and soccer."

"One of the great challenges for Michelle Obama is she is going to have to juggle many balls, wear many hats," historian Robert Watson said.

"She's going to have to be a wife, a mother, but also the first lady. And this is a woman who is used to having a very successful, high-powered career and it's an enormous challenge."

He added: "My sense is that Michelle Obama probably comes better prepared to handle these challenges than any first lady in history. The reason is that she has sort of been super woman."

"First lady" is an unofficial title bestowed on the hostess of the White House. Helped by a staff of around 100, she has many largely ceremonial duties and accompanies the president to state functions and on trips.