The Countryfile star, 44, announced her pregnancy in September, three years after her difficult pregnancy with son Zephyr.
But now she and partner Gerard Cunningham, 55, are preparing for double joy after she began IVF at the end of 2013.
Julia told HELLO! magazine: "I'm completely overjoyed. The prospect of having two for one is just incredible. Starting later in life having children, I didn't ever think I would have a big family.
"But when we found out at the first scan, we were gobsmacked. It was like, 'OK, deep breath'."
Julia thought she'd never have children after she was diagnosed with endometriosis, a painful condition affecting the womb.
But she said: "Zeph was the unexpected miracle and I was thrilled and happy to have him. It's taken a long time to get to this stage so the prospect of going from one to three is utterly amazing."
She added: "Gerard is incredibly supportive. He knows what I've been going through and said if he could do it, he would have.
"But he's equally passionate about siblings for Zeph - he's one of eight. There was never a moment when he said, 'Oh God, it's twins'.
"He was like, 'Brilliant'. If he could squeeze in four more, he'd be happy."
When she announced her pregnancy in September, Julia said: "It's no secret that our son Zephyr was a longed-for baby, and we are thrilled at the prospect of having a brother or sister for him.
'As an older mum I feel incredibly lucky. I've reached the three-month mark and am keeping as fit and healthy as possible."
Julia had her first child at the age of 41, two years after she began dating property developer Gerard.
But she was concerned she'd never have children after being diagnosed with endometriosis.
There were further problems when she was diagnosed with pre-eclampsia, which is characterised by high blood pressure and can be dangerous to both mum and baby.
Zephyr finally arrived after Julia was induced, an experience she described as 'traumatic towards the end'.
Julia's new prime time ITV show The Wonder of Britain will be shown early next year.
The full interview appears in this week's Hello! magazine - on sale now.
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