The 24-year-old, who is expecting her baby with boyfriend Simon Konecki, has been hiding her bump from the public by keeping a low profile and wearing baggy clothes.
"Adele has barely left the house in recent months, so she could keep this to herself for as long as possible. She stopped exercising a few months ago, and since then it's been all about the baby," a source said.
36-year-old Konecki, who has a five-year-old daughter from a previous marriage, has been dating Adele for a year now and since then they have been keen to have children.
It has also been rumored the couple could have married in secret, after Konecki was seen wearing what looked like a wedding ring when he attended the Ivor Novello awards with Adele earlier this year.