Molly and Arthur were fed up with the twins always pranking so they decide to pull a prank. On their 8th birthday, Arthur patted Molly's stomach and announced, "We're having another baby."
Everyone who had known Fabian and Gideon Prewett knew that Fred and George Weasley were exactly like them. They were always pulling pranks and confusing people. It got to the point where Molly and Arthur couldn't handle it anymore. So they decided to pull their own prank. Their plan was to announce that they were expecting another baby, but because the twin's birthday was so close, they decided that they would do it as a April Fool's/birthday surprise prank.
When April first came around, the Weasley parents weren't entirely sure what they were going to say so they agreed to improvise.
Walking into the kitchen after getting ready for work, Arthur quickly made his way over to the table and sat down next to his wife, who was happily eating with the kids. Before anyone could say something, he softly patted Molly's stomach.
"Kids, your mother and I have an announcement to make," he said as he as he kept his hand on Molly's stomach long enough for all of the kids to notice. "We're having another baby," he said after a few seconds of silence.
"Another one?" both Fred and George asked, equally as surprised as their siblings.
"Yep," Arthur said as he tried not to laugh. "You're due, what, in December?" he asked Molly.
"Mhm, December the twenty-first," Molly replied as she also tried not to laugh.
"Congratulations," Bill said awkwardly as he looked over at his parents.
"Yeah, congratulations," everyone chorused.
After about an hour, Molly couldn't take it anymore, she had to spill the beans.
"Kids, can you come into the living room, please?" she asked.
She knew that Arthur would be disappointed about not being there to break the news to the kids but she felt too guilty to wait until later that night.
"What's wrong, Mum?" Bill asked as he walked into the room, followed by his siblings.
"Well," Molly said as she looked around the room, making sure all of her kids where there. "I'm not actually pregnant. April Fools," she said as she let a soft smile appear on her face.
"What?" Fred and George asked at the same time.
"April Fools," Molly repeated as she started to laugh.
"Good one, Mum," Bill said once everything registered in his head.
"Yeah, good one," Charlie and Percy repeated as Percy walked up stairs to fetch one of his books.
The rest of the day went on as if nothing happened (well, except when Arthur found out Molly had told the kids).