The Glaswegian singer thanked NHS staff for all their efforts with the vaccine rollout and said that she was "grateful" for being double jabbed.
Posting a picture of her lateral flow test on social media, she said: "Not quite the Christmas present I was hoping for but thankfully me & bump are now feeling a bit better.
"So very grateful that I had been double vaccinated.
"Thank you to our incredible @NHSGGC & all those responsible for the brilliant vaccine roll out across Scotland."
McManus, who won the second series of the talent show in 2003, is expected to welcome her new addition early next year with husband Jeff Nimmo.
They tied the knot four years ago and have a one-year-old son, Harry.
The singer rose to fame 16 years ago as the champion of the second and final series of Pop Idol.
Following her reality TV show success, McManus enjoyed a number one with her debut single All This Time.
She released her first and only album, The Meaning Of Love, the following year but was then dropped by her record label a short while later.
Over the years she has made a number of TV appearances, including in a special of Gillian McKeith's You Are What You Eat, and from 2009 until 2011 she hosted STV lifestyle programme The Hour.