The "Big Little Lies" star took to Instagram over the weekend, March 16-17, to share a copy of the cover of America's OK! magazine, which featured side-by-side images of the smiling actresses.
"Jen & Reese Baby Bombshells!" read the headline. "Shock & Joy for Hollywood BFFs (best friends forever)".
Making it clear there was no truth to the claims, Reese captioned the image, "Hey @jennifer.garner! Can we raise our imaginary babies together?"
"We are going to be the cutest imaginary family!!" Jennifer replied as she played along.
"I'll just go ahead and move in now...," she added, alongside a series of laughing and heart emojis.
Their celebrity pals also got in on the fun, with Reese's "Cruel Intentions" co-star Sarah Michelle Gellar quipping, "Only if I get to imaginary baby sit," to which Witherspoon asked, "Will you be the imaginary godmother?".
"That's so amazing! Congrats to both of you! How exciting!" she commented, while comedienne Ali Wentworth added, "I'll throw the shower!"
Reese and Jennifer are both already mothers to three children. The "Legally Blonde" actress, who turns 43 on Friday, March 22, shares 19-year-old daughter Ava and 15-year-old son Deacon with ex-husband Ryan Phillippe, while she also has a six-year-old boy named Tennessee with husband Jim Toth.
Meanwhile, Jennifer and her ex-husband Ben Affleck are parents to girls Violet, 13, and Seraphina, 10, and son Samuel, seven. She is has been romantically linked to businessman John Miller since the autumn (18).