A source with direct knowledge tells us Kylie is still in the "very early stages" of the pregnancy and does not yet know the sex of the baby ... who will make 3-year-old Stormi, a big sister. We do not yet know when the baby is due.

Kylie's made it clear in recent years she didn't want Stormi to be an only child ... saying she felt pressure to have another child in 2020 but wasn't ready for it just yet.

Kylie famously spent her first pregnancy in hiding, mostly staying cooped up inside her estate, so it will be interesting to see how the billionaire decides to handle her second pregnancy.

Kylie's been sparking pregnancy rumors all summer ... first when she wore baggy clothes in Idaho and again when eagle eyed fans noticed her sushi order didn't include raw fish.

Page Six was first to report Kylie was expecting.

Caitlyn Jenner also seemed to let the cat out of the bag Thursday when she said she just learned one of her kids is expecting. Caitlyn's son, Burt, says he's expecting a third child with his wife, Valerie, but now we know Kylie will be bringing a new kiddo into the world too.