The pro dancer and choreographer took to Instagram to make the announcement and posted a black-and-white video of her gender reveal party at their Utah home.
Set to the song "Somewhere Only We Know" by Rhianne, Lindsay and Sam were blindfolded in the backyard, and once a smoke cannon was released, the video changed to color and revealed pink smoke in the air.
"IT'S A...........GIRL!!!!" Lindsay captioned the video with red hearts and smiley face emoticons.
"We are beyond excited to bring another sweet girl into our family and so so happy that Sage will have a sister."
Lindsay later reflected on her pregnancy via Instagram Stories, telling her followers, "I am so excited that the news is out!"
"You guys, we are having another girl!... I obviously would've been happy with either a boy or a girl, I would be stoked, but I am so excited that Sage is going to have a little sister," she gushed.
"I can't imagine a life without sisters, so I'm so grateful that they will have each other. It's so crazy -- I'm going to have two little girls!"
Earlier this week, Lindsay, who is entering her second trimester, made a prediction about her baby's sex -- and she turned out to be correct.
"I feel pretty much exactly the same [as I did with Sage]... The nausea level is exactly the same and I'm treating it the same way... If I'm not snacking every hour, it makes me more sick," Lindsay told Us Weekly when comparing her two pregnancies at this stage.
"And I'm gravitating towards the same foods as I did last time. I loved fruits, vegetables, salads, and I didn't really want meat -- which is so weird, because I'm a big meat girl. That is my favorite food group... So it's very, very similar."
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Lindsay concluded, "That's the only indicator where I'm like, 'Maybe it's a girl,' but then I'm like, 'I have no idea,' I really don't. I was so wrong in my guesses about what Sage was, so I just feel like I can't even trust my instincts on that part."
Lindsay said at the time she just wanted a healthy baby but she grew up with only sisters.
"All I know is sisters because that's all I've ever had. So I do hope for that for Sage, but I do think a brother would be really fun for her as well," Lindsay explained.
"Either way, I'm so glad because she is going to be so good to them. She has been the best little cousin to my niece and nephew. She is so sweet with them... and understands she has to be gentle with them."
"Doing what is best for my daughter is always going to be at the top of my list!" Lindsay wrote on Instagram before the new season premiered in September.
"As many of you know, my family and I live in UT and in order to do DWTS I have to move myself and my daughter Sage out to LA and leave behind my husband Sam who works full time here in Utah."
She continued, "And there is a lot of sacrifice that has to be made that unfortunately just did not feel right for us this time around. We exhausted every option we could think of to make it work but at the end of the day, none of the options felt good in my heart or felt like the right thing for our lives right now."
Lindsay also confirmed at the time how she and Sam were trying for their second child.
"We are also trying to grow our family and that in itself is a complicated logistical thing as I'm sure many of you understand," Lindsay wrote.
"All this being said it's so important to me that you understand none of this has anything to do with my love for the show as that will NEVER change and it also doesn't mean that this is the end for me and DWTS."
"Although this decision has been so hard to make," she added, "I feel so much peace about it and know that it is the right thing to do."
Lindsay announced she was pregnant with her first child in May 2020, just one month before she and Samuel celebrated five years of marriage.
Lindsay and Samuel were high school sweethearts who tied the knot at the Salt Lake Temple in Salt Lake City, UT, in June 2015. Approximately 150 guests reportedly attended their reception at the Sleepy Ridge estate.