"Pulled it off everyone! Happy to celebrate you and our babies today," Arie captioned the slideshow.
Arie and Lauren, with her baby bump on full display in a light peach maxi dress, posed in front of a "Baby L & S" sign, which appears to hint at the twins' names, as well as inside a white bounce castle set up for the couple's 22-month-old daughter Alessi Ren.
Arie also posted a picture of some delicious food and Lauren smiling with a few of her girlfriends.
Lauren commented on Arie's post, "Still can't believe I never caught on! There were so many signs."
On her own Instagram account, Lauren posted similar pictures, including one of her munching on pink cotton candy alongside a pal who had a stick of blue cotton candy in hand.
"Never been more surprised than I was walking into my backyard today. love all of you ppl so much & @luyendyktwins. We can't wait to meet you!" Lauren gushed.
On the Instagram account the couple had created for their, Arie provided a little more insight into why he decided to surprise his wife with a baby shower.
"29 weeks! Today we spent the day celebrating you two. I was so happy to surprise Mom with a baby shower and she had no idea!" Arie began.
"This past week was a tough one for her so I thought this would be the best way to put a smile on her face. You have so many people out here that already love you so much and can't wait to meet you! So this week I got to finally see your little faces and you two look so dang cute."
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Arie continued, "I have to say I see a lot of your big sister in those pictures but trust me she's cute so that's a good thing. Also baby girl you are facing down now so we are crossing our fingers your brother will flip too!"
"That's all for now but keep growing and we'll hopefully see you in 9 weeks! Love you, Dad," he concluded.
Lauren finished Arie's season of The Bachelor in second place when Arie actually chose Becca Kufrin as his winner and popped the question to her during the Final Rose Ceremony, which aired in March 2018.
But weeks after getting engaged, Arie decided to dump Becca because he claimed he was still in love with Lauren, and so he begged Lauren for a second chance.