"Baby McAllister coming Jan 2021!!!!! We can't even contain our excitement and joy right now," Witney wrote alongside a photo of herself embracing Carson while holding sonogram photos and a positive digital pregnancy test.
"We found out together with @clearblue, which was so surreal and special! Getting our results in words made this whole thing that much more real! We love this little bean so much already and we can't wait to start this beautiful chapter in our lives!"
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Witney added the following hashtags to her post, "#15weekspregnant #mcallisterbaby #clearbluepartner #clearblueconfirmed."
Witney, who married her high-school sweetheart in Salt Lake City on New Year's Day in 2016 after dating for four years, told Us Weekly in October 2019 that she had baby fever and wanted to have a child with Carson "so bad" soon.
"We're not really putting a timestamp on it, just when we feel ready," Witney told Us at the time.
"I get so baby hungry though. Like, every time I see a baby it breaks my heart. I want one."
Witney previously worried, however, about one "big issue" in regards to getting pregnant with Baby No. 1.
"Can you start a family and still have a career and still dance?" Witney questioned, according to Us. "Peta has been a really good mentor to us through what we would do when we eventually have kids. She's been awesome."
Witney had also revealed she "wants a lot of kids" but "four is going to be the [maximum]" for her family.
Due to COVID-19, Season 16 of The Bachelorette -- which was originally supposed to have premiered in May and air on Monday nights this summer -- is now scheduled to air Tuesday nights on ABC this fall instead.