The ex-American footballer, 35, is said to have popped the question over Christmas and the couple "can't wait" to plan the wedding after Nadine, 28, gives birth in February.
''Nadine is on cloud nine after Jason went down on one knee over Christmas and proposed," a source told The Sun on Sunday.
''She is a traditionalist at heart and has told friends they will become a 'proper family' after they tie the knot. She hasn't started planning the wedding yet as obviously she is set to drop any second now. But she can't wait to organise a big white wedding once she's settled into motherhood.''
Shortly after the proposal, the former Girls Aloud band member announced on Instagram that she was expecting a baby girl. Nadine posted a photo of a balloon shaped as a stork, with the words 'It's a girl' stamped on the side.
Speculation has started on whether Nadine's former band mates will attend the nuptials, with the insider adding that it was "doubtful Cheryl Cole would turn up given their history."
In an exclusive photoshoot with HELLO! magazine in October, however, Nadine added that she would like to have a reunion in the future. "Hopefully one day we will get back together to do something else," she said.
This is the second time that Nadine and Jason have got engaged, after the brunette beauty ended their first engagement in June 2011 to focus on "more me-time" before committing to marriage. The couple first started dating in autumn 2008.