Liv is already a mum to nine-year-old Milo with her ex-husband, British musician Royston Langdon. Sources have said the star is "delighted" by the news and that she was "really trying for another baby".
She is currently dating British sports agent Dave Gardener – a best friend of David Beckham.
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Liv Tyler and Dave Gardner
The couple have been pictured together out and about in the last four months after being introduced by mutual friend Kate Moss. In July they joined the supermodel and her husband Jamie Hince on holiday in Spain.
Liv has made no secret of her desire to have more children. In a recent interview with the Sunday Telegraph's Seven magazine, she said, "I'm 100 per cent planning on it.
"If the stork could just drop it off on my roof, I'd be so happy – I'd have, like, 20."
Liv Tyler has made no secret of her desire to have more children
The actress is less certain about tying the knot for a second time, though, explaining that her perception of a fairytale ending has changed over time.
"That's something interesting that happens in your thirties. You're not in the stage anymore of princes and happily ever after.
"It's a different stage of acceptance and realisation about the realities of love and relationships. Forever is a long time."
Liv was married to Royston from 2003 until 2008, and they welcomed their son in December 2004. Dave has a seven-year-old son, Grey, with his ex-wife Davinia Taylor.