"Can't wait for you," the 29-year-old captioned the black-and-white pic, adding a heart and smiley-face emoji.
Us Weekly broke the news in July that the Arthur George sock designer and his fiancée, Blac Chyna, are having a daughter. The pregnant model, 28, already shares son King Cairo, 3, with ex Tyga. The rapper, 26, and Rob's half-sister Kylie Jenner have been off and on since 2014.
"I'm just so attracted to how she raises her son, King. That's what attracted me to her initially," Kardashian said of Chyna during a Facebook Live on September 8. "She was just a great mom … and [that] led me to want to start a family with Chyna. It's something I've always wanted to do."
The Rob & Chyna stars have come a long way with his famous family. Since the couple announced their pregnancy, the Kardashians have looked past Chyna's feuds with Jenner, 19, and Khloé Kardashian. Last week, Kim Kardashian and Kanye West even gifted the future spouses with Pablo-branded baby gear including onesies and shoes.
"I couldn't really say anything. I couldn't really feel comfortable about talking to any of my family members obviously about the situation," the dad-to-be recalled during the Facebook Live. "It was very, very complicated, but my happiness outweighed everything, honestly, at that point in my life. She was making me feel very comfortable. I didn't feel judged in her home. I was on my health trip, and I just felt very good."
Sign up now for the Us Weekly newsletter to get breaking celebrity news, hot pics and more delivered straight to your inbox!