23/03/2015 15:01 GMT | Updated 23/05/2015 10:12 BST

Elle Macpherson, 51, 'Having Baby With Surrogate'

MIAMI, FL - MARCH 13: Elle Macpherson hosts the 2015 Power of the Purse Luncheon to benefit The Women's Fund on March 13, 2015 in Miami, Florida. (Photo by Manny Hernandez/Getty Images)

Elle Macpherson – who turns 51 this week - is having a baby via a surrogate, according to Australia's Woman's Day.

The supermodel and business woman – already mum to sons Flynn, 17 and Cy , 12 – has opted for surrogacy after trying to get pregnant with her husband Jeffrey Soffer over the last 18 months.

Elle and property mogul Jeffrey, 45, married in 2013 and have been keen for another child ever since. Her sons' dad Arki Busson, a financier, has a three-year-old daughter with actress Uma Thurman.

A source told Woman's Day: "They realised after a few months that having a baby naturally was a lost cause and they barely discussed adoption – they agreed on surrogacy quite quickly.

"Elle is thanking her lucky stars she had the foresight to freeze her eggs years ago. She figured they'd come in handy."

According to the magazine, the surrogate, is in her 30s but no other details have been revealed.

The insider told the magazine: "They're keeping the identity of the surrogate a secret.

"It's what she wants and it's what they want. But I do know she's in her 30s and has two children of her own."

Elle, judge of Britain and Ireland's Next Top Model, and Jeffrey began dating in 2009 but split in April 2012 after three years together.

They then reunited seven months later when Jeffrey was injured in a helicopter accident.

Elle told Channel Nine last year that her family is her 'greatest achievement' – but the logistics can be challenging.

Elle, who shares custody with her sons with ex Arpad Busson, said: "My step-children live in Aspen, Colorado. My children live in London and my husband lives in Miami."