"Today was supposed to be our wedding day... But we decided to make this day special nonetheless," Ben, 31, captioned a slideshow of photos.
The photos featured Ben placing his hand on Stacy's baby bump as the couple posed with their two dogs, including Ben's dog Zeus, who was wearing a "Promoted to Big Brother" sign around his neck.
"When we decided to postpone our wedding we talked about the future and how this would effect [sic] it. Naturally we started talking about a family. I've always known I wanted to be a dad but didn't know when I would be ready," Ben continued in his post.
"Several doctors told Stacy based off certain health factors it was going to be extremely difficult for her to get pregnant, and the chances would continue to drop the more time went by. Long story short... out of some miracle (or powerful swimmers) we started trying and got pregnant on the first try! WE ARE EXPECTING!"
Ben admitted he and Stacy are "both terrified and excited at the same time."
"From other parents we've talked to, that 'sounds about right.' We were not gonna let this virus stop [us] from moving forward!" Ben wrote, referring to the ongoing coronavirus pandemic that is the reason for the pair's postponed nuptials.
"I'm so excited to be having a baby with the love of my life and I know she's going to be the best mom! Our fur babies are gonna be in for a rude awakening but I'm sure they will be great as well."
Stacy, for her part, took to Instagram with a lengthy message of her own announcing she's pregnant.
"Happy (what would have been) wedding day! Of course there are mixed emotions and sadness of what today would have been..... But today we get to share even more exciting news with everyone!" Stacy began.
"We get to bring a little baby in to this world February 2021 and love on it for the rest of our lives! Something I never, ever thought was possible for me!"
"I was told years ago I couldn't have kids, then when Ben and I began dating I knew becoming a dad was something he really wanted. I began getting some tests done again just to see where things stood, I was then told I could have kids but my numbers showed extreme low ovarian function," she explained.
"I wrote off being a mom for years and the thought of what struggles lay ahead made be push the idea away even more. When COVID-19 happened and we decided to push our wedding back a full year, @benzornlife looked at me with those baby fever eyes and I couldn't say no."
Stacy said she realized "waiting another year to try" was out of the question because it "would lessen whatever chance we had, even more."
Stacy said she and Ben bought certain products and did research on how to understand her body and the beginning of her pregnancy journey.
"I was set and mentally preparing myself for what could be a long, hard road. I tracked things for about 5 months and we finally decided to give it a go in May," Stacy shared.
"Ben and I were pregnant by June. First. Dang. Try! I'm still in shock and still don't believe this is really happening. I'm still struggling with processing all of this happening so fast, and coming to terms with how much life will change."
Stacy admitted, "I never thought being a mom was going to be a part of my story. I've been battling a lot of anxiety and fear recently. But there is no one I'd rather share this incredible journey with."
Stacy said all the "love" and "support" she has received is "overwhelming."
"And I can't wait to have baby Z apart of our wedding next year!" Stacy gushed. "#baby #wedding #love #parents #omg #puppiesofinstagram #tahoe #family #bestfriends."
Although Ben posted a photo of the pair in early October, he didn't introduce Stacy as his girlfriend and go Instagram official with her until late October 2017, when Ben posted a sweet photo with the dental assistant at a pumpkin patch.
"How did I get so lucky to have you FALL into my life," Ben wrote, adding a heart emoji and a "birthday fun" hashtag.
"She said yes! Friday 8/23 I asked this beautiful women to marry me on the shores of Lake Tahoe, CA a place that's always meant so much to us," Ben captioned an Instagram photo at the time.
"@ssantilena you are my best friend and I'm so lucky I get to call you mine! I couldn't be more excited to spend my future with you! Let the Wedding planning begin!"
And Stacy confirmed the news of her engagement on Instagram, writing, "On Friday August twenty-third at approximately 6:30pm the most amazing man in world became mine forever! I still pinch myself everyday that he is my boyfriend, now I'm trying to wrap my head around him as my fiance! #ItsAlwaysBenYou."
"I never want to imagine this life without him now that I know what life is like with him!" she added. "@benzornlife thank you for making me the happiest girl in the world! Im so excited for forever with you!"
Ben found fame when he competed on Kaitlyn Bristowe's season of The Bachelorette in 2015. He looked for love again on Season 4 of Bachelor in Paradise two years later but fell short.