The couple got engaged in September.
"Marriage and kids are a natural part of a strong relationship filled with love and respect," he captioned an illustration of two lions with a cub in between them.
Asghari added, "Fatherhood is something I have always looked forward to and I don’t take lightly. It is the most important job I will ever do."
Prior to Asghari's post, Spears shared a photo of a cup of tea and flowers.
"I lost so much weight to go on my Maui trip only to gain it back 🤷🏼♀️🤷🏼♀️🤷🏼♀️ … I thought 'Geez … what happened to my stomach???' My husband said 'No you’re food pregnant silly 🤪 !!!'" she wrote, referencing Asghari, 28, whom she's yet to wed.
"So I got a pregnancy test … and uhhhhh well … I am having a baby 👶🏼," Spears, 40, continued. " … 4 days later I got a little more food pregnant 🤰🏼🙈🙈🙈 It’s growing!!! If 2 are in there … I might just loose it 😬😬😬😬😬😬"
Spears added that, in the wake of her news, she "obviously won’t be going out as much due to the paps getting their money 💴 shot of me 📸 like they unfortunately already have …"
The singer additionally noted that she'll be "doing yoga 🧘♀️ every day" and "spreading lots of joy and love 💕!!!"
Spears, who is already mom to Sean Preston, 16, and Jayden James, 15, with her ex-husband, Kevin Federline, left fans confused with her post, as she referenced being pregnant with a baby and with food. ET has reached out to Spears' lawyer and Asghari's rep for comment.
Spears and Asghari announced their engagement in September, after meeting in 2016. During Spears' court testimony in June, she revealed that she wanted to get married and have a baby, and alleged that her conservatorship prevented her from doing so.
When ET spoke with Asghari last month, he gushed about life with his fiancée.
"Everything’s amazing," he said. "We’re so excited to start a new chapter, and everything’s so positive from here on out."