Rob Kardashian and Blac Chyna are about to expand ...  we've learned she's pregnant!

Sources close to both Chyna and Rob tell us Chyna's several months along -- and it's not one of those, "Oh crap, you're pregnant" situations. We're told they're both super excited. 

The baby will be Rob's first and BC's second ... she's got 3-year-old King, whose dad, of course, is Tyga. 

Congrats.11:25 AM PST -- Rob and Chyna just confirmed the baby news on Instagram with a pregnant Chyna emoji. Tyga also sent his well wishes, writing "Congratulations" under Chyna's announcement ... Kylie "liked" the post too.