It is thought that the father might be her former fiancé Jason Bell. Earlier this year she reportedly rekindled her romance with the American footballer, 35, who she dated for over three years.
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The couple announced the end of their one-year engagement in June 2011, with Nadine confirming the news via her Twitter account. "A few weeks ago Jason and I broke up," she told fans. "We are great friends and decided we are better this way. I'm in NYC working & I love all your support x."
But in March this year, the pair sparked speculation they had reconciled, with Jason travelling to the UK to see Nadine perform at the O2 Arena on Girls Aloud's farewell tour.
Nadine will be the first member of the band to have a baby, and she hinted to her Twitter followers a few days ago that she had some news. "I have some Craic for you Xx," she wrote.
The Irish singer now lives full-time in LA with her family, but regularly returns home. In April, she announced that was working on the follow-up to her debut solo album Insatiable, which was released in November 2010.