The couple shared the sad news on their social media on Thursday. The actor's wife had been six months pregnant with their first child before they lost their baby girl.
Calling her his "Wonder Woman," Kellan explained on Instagram that it has been a "crazy rollercoaster of a week with a lot of emotions."
"Taking heartbrokenness to a whole new level but Grateful for these past 6 months and the journey itself, " he wrote alongside a black-and-white photo of Brittany showing off her baby bump. "In life we might not get the answers to all of our questions but we will always remain fairthful! [sic] God will restore. I love you @brittanylynnlutz."
"Thank you all for all your love, respect in this private time, understanding of this situation, and utmost support!" he continued. "Just know we love you all and appreciate you all and we process all differently. Can’t wait to see our baby girl in Heaven when God calls us home?."
Brittany posted the same photo, elaborating on the tragic news.
"It was my absolute honor and pleasure to be your mom these last 6 months. I did my best and it was an absolute joy seeing your little face all those times on that screen and feeling your tiny kicks," she began. "I don’t know why it happened the way it did, but part of me finds so much peace knowing you never experienced pain or heartache and never will. You’re in the arms of Jesus now and one day we will get to meet you for real. Until I see you in heaven... your mommy loves you so much. ?."
She explained that she is not ready to talk about what happened and is not sure when she will be. "But I can say I am SO grateful for the most amazing husband who’s been by my side the entire time," she wrote. "I have the best most supportive family. The prayers from friends have meant everything."
"Thank you for respecting all of our privacy right now. Gonna take some time away to process and heal," she concluded.
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‘FBI: Most Wanted’ Actor Kellan Lutz Talks Memories of Creating ‘Twilight’ Saga (Exclusive)
ET spoke with Kellan in December during an FBI: Most Wanted set visit, where he had expressed how excited he was to become a first-time dad.
"Brittany and I, we're pregnant and it's our first one and we're really excited for it. It's been on my heart for a long time," he shared. "I've always had a father's heart and I come from a separated family. My parents were divorced and so never felt complete really."
"I'm really looking forward to teaching my children the stuff that I learned not to do and what to do and I want a big family," Kellan added. "I really want to have a bunch of kids."
He also praised Brittany, saying she is the "nicest person I know and she's my best friend and I'm really grateful that I have that with her."