The 'Insatiable' singer is expecting her first child with Jason Bell and her former Girls Aloud bandmate admits she and the rest of the group were stunned to hear the news.
Sarah said: ''Who's have thought she'd have been the first one of us all to get pregnant. Me and Cheryl were like, 'Oh my God, can you believe it?' It is good, though I couldn't believe it when I saw those pictures! She's actually got a bump. I thought, 'This is weird'.''
Sarah - who is dating producer Mark Foster - admitted she only now feels like a ''grown-up'' because she isn't in the band any more.
She explained: ''I'm actually starting to feel like quite a grown-up. I suppose you don't really have the chance to grow up when you're in a girl band. When there's five of you, you think you can get away with murder but now the pressure's on! I do feel more responsible.''
And the blonde beauty is amazed at how The Saturdays - whose member Frankie Sandford is pregnant with her first child, while Rochelle Humes and Una Healy are both married with daughters - juggle their careers with marriage and parenthood.
She added to Britain's OK! magazine: ''Look at The Saturdays, married or wifed up with kids and they're still so young. They're still girls in my eyes.
''I love The Saturdays but it's weird, dancing around on stage with a big pregnant belly. I don't know what it would have been like for our image if one of us was bopping around on stage with a bump. But The Saturdays have done really well. It's nice they're all happy and settled, but for us it was all career, career, career.''