"Fixer Upper" stars Chip Gaines and Joanna Gaines celebrated 15 years of marriage on Thursday. Chip, 43, marked the occasion by penning a short poem to Joanna, 39, on Twitter. "15 years.. where'd they go. 15 years.. I'd like to know. 15 more, to God I pray. 15 more just to make her day," he wrote. "Happy anniversary sweet girl!" Joanna dedicated a sweet post to Chip on Instagram, sharing a throwback photo from their wedding in Waco, Texas. "It's been a good fifteen years @chipgaines," she captioned the picture, adding a heart emoji. "Here's to fifty more..." Chip and Joanna are parents to 13-year-old son Drake, 12-year-old daughter Ella, 12-year-old son Duke and 10-year-old daughter Emmie, and announced in January they're expecting their fifth child. RELATED LINK: '90 DAY FIANCE' COUPLES NOW: WHO'S STILL TOGETHER? WHO HAS SPLIT?! (PHOTOS) Joanna said in May her pregnancy came as a surprise. "When I saw the results of the test, my jaw dropped," the star said. "Chip was in the driveway about to leave when I called him back and said, 'We're pregnant!' He was so excited. We're both just so excited." "Every single time [my kids] see my belly they have to rub it," she added. "I think it's sweet because they're older now, and I can see them wanting to be a part of this." "Fixer Upper" ended in April after a five-season run on HGTV. The show followed Chip and Joanna as they renovated homes in the Waco area.