The 'mistress', who wishes to remain unnamed, told In Touch magazine that carrying 27-year-old Josh's baby would have been 'the worst thing that could happen to her'.
She said: "I would never want a man who's a sexual predator to be the father of my baby.
"He's a monster. All I was thinking was, 'Oh my God, I'm pregnant and i'm carrying Josh Duggar's baby'."
In May, father-of-four Josh admitted to sexually abusing five young girls when he was 14 and 15. Four of the victims were his sisters.
He later confessed to cheating on his wife and being addicted to pornography, after dating site Ashley Madison suffered a security breach.
Following the two scandals Josh, who was raised as conservative Christian, is believed to have checked into rehab.
His alleged lover added: "I hope he gets the help he needs and rehab so that his wife and children will remain safe, but I fear no faith-based rehab ail fix him."
The family shot to fame on the TLC show 19 Kids and Counting, which was cancelled after Josh's scandals came to light.