Ms Belfield, 36, a cleaner from Stoke-on-Trent, submitted her first benefit claim when she was pregnant in 1999. When her daughter was stillborn, she substituted her baby with her nephew on benefits' paperwork and so continued to receive payments.
She claimed £45,061 as the Department of Work and Pensions failed to spot her deception, despite her claims being reviewed six times. She was jailed for six months at Stoke-on-Trent Crown Court.
Nicholas Tatlow, prosecuting, said: "In December 1999 the defendant made a proper claim on the basis that she was expecting a child. In March 2000 she failed to notify the department that her child had sadly been stillborn.
"Instead she claimed to be the mother of a boy that was a real child, but not her's. On March 24, 2000 the department received correspondence from the defendant to the effect that she had had a child.
"In April 2000 a review form was submitted in which she named the child, giving a date of birth of March 1999."
Ms Belfield pleaded guilty to failing to notify the DWP of a change in circumstances.
Heather Drew, mitigating, said her client had been psychologically damaged by losing her daughter:
"She went through a very emotional and stressful experience. Although there is no diagnostic mental heath issue, a doctor said she would have been psychologically affected by this.
"Perhaps she felt she wanted to have a child and it was put down on the form, then it went on too long and she couldn't turn the clock back. It's something that she doesn't ever seem to have got over. She continues to visit the grave of her baby, and she also lost her partner in 2008."
Jailing Belfield, Judge Mark Eades said: "You deliberately made a false claim in respect of this boy. You had no right to claim for him at all.
"You knew when you entered his name on the form that it was a false claim, and you kept the pretence up for a long time. I take the view that such blatant dishonesty must mean an immediate custodial sentence.