The 31-year-old, who stars in the new Netflix show, "You," shared the news on Instagram, where she posted an image of the sonogram with a broken-heart emoji.
Directly after that image, she included a text post that read, in part: "We all have to deal with various struggles and challenges in life. And sometimes it's easier to only showcase the good times on social media, which is what leads many people to criticize it for its lack of authenticity. Having so many people follow me on Instagram and read my posts is both incredibly humbling and hugely uplifting."
But then she added, "The support and affection that so many of you show me lifts me up during even my darkest days, one of which happened last year after I miscarried and lost the child of my hopes and dreams."
Mitchell, who has kept her dating life relatively private, also looked to the future in her post.
"In the spirit of the new year, I think that we need to remember that we are all on this journey together — in good times and in bad — and to remind ourselves that we seldom really know or understand the struggles and hardships that other people are going through," she wrote.
"So, for 2019, let’s all try to be a little more compassionate, empathetic, patient and thoughtful with each other," Mitchell continued. "This sounds like a great resolution to me. I hope that you agree. Happy New Year everyone!"