Arie and Lauren welcomed their baby twins, a boy and a girl, via C-section on June 11 and were able to bring their son home on Tuesday; however, Lauren shared on Instagram her newborn daughter had to stay behind in the hospital.
Lauren posted a photo of the siblings on her Instagram Stories before leaving the hospital with her baby boy and wrote, "2/3 of my heart right here."
"Today is so bittersweet, we get to leave the hospital," Lauren, 29, continued in her post on Tuesday. "But little sis has to stay behind for now."
As a video played of Lauren rubbing her baby girl's hand, she wrote, "I've never felt heartbreak like this. Pls say prayers for our girl."
Arie, 39, and Lauren shared a video of their babies Monday on YouTube and a nurse revealed to the couple what was happening with their daughter from a medical perspective.
"Babies transition really quickly during the first 12 hours of life," a nurse explained to Arie and Lauren as Lauren rubbed the newborn's back.
"Some babies need it for a couple days. It's just depending... [on] when the doctor comes over if she's still needing respiratory support for a couple more hours. We might get a chest X-ray just to see a better picture of what's going on."
Arie and Lauren's son -- who only spent "a few hours" in the NICU after his birth -- was "doing so good" at the time, while his sister gained strength and "got a little support" in the NICU.
Arie and Lauren are also parents of a two-year-old daughter, Alessi, who was born in May 2019.
Alessi apparently got to meet her new brother for the first time on Tuesday once Arie and Lauren returned home from the hospital with one of their twins.
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"Already loves her brother so much," Arie posted photos of Alessi were her younger sibling.
"Thank you everyone for the kind words and for those that watched our vlog on YouTube. Emotional few days but Alessi meeting her little brother today was cuter than words could describe. We'll post more soon just resting."
Lauren also shared a photo of her twins on Instagram and just captioned it, "6.11.21. @luyendyktwins."
Arie shared the first photos of the newborns with their mom on Monday, when he gushed about Lauren.
"Never been prouder of my wife than in this moment," Arie wrote on Instagram. "She is the strongest woman I know and I'm lucky to be on this incredible journey with her. We are proud parents of a healthy boy and girl!"
Arie surprised Lauren with a backyard baby shower for their twins in late April when Lauren was 29 weeks along in her pregnancy.
"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 Instagram after the big bash.
As for where they plan to live with their three children, Arie and Lauren recently bought a house in Hawaii, although they won't stay there "full-time." The pair reportedly plan to split their time between Hawaii and Arizona.
"For anyone who has been to Arizona during the summer, you know it's kind of miserable here," Lauren shared with the magazine in May. "We just wanted to have a good escape, and I've always wanted to be near the beach."
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.
Arie proposed marriage to Lauren in March 2018 on The Bachelor: After the Final Rose after Season 22 aired on ABC.
The couple then moved into their first home together in Phoenix, AZ, in April of that year.
The pair announced they were expecting their first child together in November 2018 and revealed two months later that Lauren was pregnant with a daughter.
Arie and Lauren tied the knot at Haiku Mill in Maui, Hawaii in January 2019 and then welcomed Alessi on May 29, 2019.