Hilary Duff says her son Luca is already ''warming'' to the idea of welcoming a baby sister.

The 30-year-old actress recently announced that she and boyfriend Matthew Koma are expecting their first child together, and six-year-old Luca - whose father is Hilary's ex-husband Mike Comrie - is slowing getting used to the thought.

Appearing on 'The Late Late Show with James Corden', the 'Wake Up' hitmaker shared: ''The beginning was a little ... he was surprised, and now he's warming up to the idea.

''I think it's a girl, but he told his friends at school the other day that he's having a sister that's part red-butt monkey, so we'll see. We never know.''

Hilary - whose marriage to Mike ended in 2016 - also revealed that Luca has already decided upon a name.

She quipped: ''We're up for any kind of suggestions, but Luca thinks Cafont Croissant is, like, the coolest name. When I just refer it as 'Cafont', and I'm like, 'That would be so cool, nobody at school's gonna be named Cafont', and he's like, 'Croissant. Cafont Croissant.' It, like, goes together, there's no separating the two.''

Hilary and Matthew announced their baby news last week in an Instagram post.

The loved-up singer shared a photo of herself and Matthew sharing a kiss while her maxi dress showed off her growing baby bump.

The post was captioned: ''Guess what guys! @matthewkoma and I made a little princess of our own and we couldn't be more excited!!!!!! [sic]''

Matthew - who started dating the former 'Lizzie McGuire' star in 2016 - shared the same photo and wrote: ''We made a baby girl! She will be as beautiful and sweet as her mother ... @hilaryduff another incredible chapter begins. [sic]''