The 'Insatiable' singer is expecting her first child with Jason Bell and her former Girls Aloud bandmate alleges the brunette beauty is exhausted.
She told the Daily Mirror newspaper: ''Nadine's getting sick of it! I know she's knackered.''
Sarah and Nadine have been in touch since the band - who called it quits just hours after they performed the final date of their reunion UK tour in Liverpool in March - split and the blonde beauty says Nadine is currently busy preparing for the festive season.
She said: ''We were texting last Saturday. She's putting up the Christmas tree and nesting. She's due in February so she's getting ready.''
Sarah - who is dating producer Mark Foster - previously admitted she found Nadine's pregnancy ''weird'' when news broke as did Cheryl Cole.
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'.''