Bust out the pink balloons -- 'cause Kim Kardashian and Kanye West are having a baby girl ... TMZ has learned.

Sources familiar with the pregnancy tell us the couple's surrogate is carrying a female bundle of joy.

We broke the stories ... the baby's due in late January by a surrogate Kim and Kanye hired for medical reasons. Kim's placenta accreta condition could kill her if she carries another baby to term.

This will be girl #9 in the Kardashian clan -- not including Kris herself -- and we've learned it bodes well for one of those ladies in particular.

We spoke with a notable child psychologist, who tells us North West and baby #3 will probably form a closer bond than Saint might with his new sis. And the reason's simple: sisters gravitate to their sisters. You don't gotta tell us ... or the Kardashians, either.

Kind of a bummer for Saint, though -- especially since Kim recently said North "does not like" him ... and doesn't seem to be getting past the meanie phase.

Oh well, girl power!