The man, who cannot be named for legal reasons, admitted two charges of rape and was sentenced to 18 years in jail at Warwick Crown Court.
The girl's mother has also been jailed for six years after pleading guilty to conspiracy to rape and child cruelty.
According to The Telegraph, the couple were caught after their doctor became suspicious when he confirmed the girl was pregnant. The girl then reportedly confided in a teacher that her step-dad had "made me do it."
"You planned to have a child together, but were told it was not possible,” Judge Andrew Lockhart QC told the couple during the trial. "You both then decided that one way forward would be for her to become pregnant and for you to raise the child.
"That was a wicked thing to do. It had no regard for her age or for the future damage it would do, or for her welfare or the welfare of any child that might be born.
"This was not just rape, but rape with the intention of ensuring the pregnancy of a 12-year-old child."
This article first published on Marie Claire.