The 29-year-old Keeping Up With The Kardashians star took to Instagram on Sunday – Mother's Day in the US – to praise his partner, who is already mum to son King Cairo, three.
Rob said: "I fell in love with her immediately because of how great of a mum she is and now I'm looking forward to starting a family with her! Thank you Chy for everything you do for me."
His heartfelt post received 170,000 likes.
Rob and Blac Chyna – real name Angela – confirmed their baby news on Friday, after it was leaked ahead of their planned announcement on Mother's Day. They started dating in January and became engaged in April.
In a twist, the father of Blac's son, King Cairo, is rapper Tyga, who is now dating Rob's youngest sister Kylie Jenner. He congratulated Blac on the pregnancy news via Instagram, where she'd shared a pregnant woman emoji.
Meanwhile, Rob didn't forget any of the mothers in his family overt he weekend, sharing Mother's Day tributes to mum Kris Jenner, grandmother MJ, and sisters Kourtney and Kim.
He said of mum Kris: "I'm APPRECIATING all the mums heavy today so THANKS to all the beautiful mums in this world ‼️ lol cuz they deserve it ,,,, once again mama I love you and thanks for putting up with your 6 crazy kids every single day and love you more than you know (sic)."
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