The Irish-born singer uploaded a snap of herself looking surprised with the caption: ‘I am having a baby. Xx’.
The father is yet to be confirmed but it has been rumoured that she has rekindled her romance with American footballer Jason Bell.
The pair were pictured together in March this year, after dating for three years and being engaged for just one before their relationship came to an end in 2011.
Jason even travelled to the UK to see Nadine perform with Girls Aloud at London’s O2 earlier this year.
The announcement marks the first pregnancy for a Girls Aloud member.
The rest of the girl group, who officially split in March this year, have yet to send their congratulations to Nadine on Twitter.
But Nadine’s fans were quick to respond to the news. ‘You’re going to be an amazing mum’, one wrote.
The shock announcement came as a surprise for everyone, as one fan explained that Kimberley Walsh would have been likely to fall pregnant first after dating her boyfriend Justin Scott for more than ten years.
Nadine revealed this year that she had plans to release new solo music but it looks as if her plans will have to be put on hold.
Congrats, Nadine!