In a post shared to her Instagram Story on Thursday, the "Smile" singer made the reveal while also making the point that nobody should feel the need to "justify" having one themselves.
"I wish people would stop posting examples of exceptional reasons for having abortions. most people I know, myself included, just didn't want to have a f---ing baby," she wrote. "AND THAT IS REASON ENOUGH! WE DON'T HAVE TO JUSTIFY IT."
"It shouldn’t have to be said and i think all these examples just play into the hands of the baddies," she added.
Allen's post comes after she joined Olivia Rodrigo on stage during her set at the Glastonbury Festival last weekend -- and performed her single "F--- You," which they dedicated to the Supreme Court justices who made the devastating decision.
"I'm sharing the stage with Lily which is the biggest dream come true ever. But I'm also equally as heartbroken over what happened in America yesterday," said Rodrigo during the set. "I'm devastated and terrified and so many women and so many girls are going to die because of this and I wanted to dedicate this next song to the five members of the Supreme Court who have showed us that at the end of the day they truly don't give a s--- about freedom."
"This song goes out to the justices Samuel Alito, Clarence Thomas, Neil Gorsuch, Amy Coney Barrett, and Brett Kavanaugh," she concluded -- before exclaiming, "We hate you!"
Allen has been open in the past about her experience with miscarriage and stillbirth, before welcoming daughters Marie and Ethel with Sam Cooper. She's currently married to "Stranger Things" star David Harbour.