12:50 PM PT -- Dakota's rep tells us she is NOT pregnant, and they were just throwing a birthday party. The rep did not say what the deal was with the pink and blue balloons.

Chris Martin and Dakota Johnson are having a baby, and the timing of their gender reveal party is pretty interesting ... considering what Chris' ex just celebrated.

Chris and Dakota, who've been dating for about a year, had a party at his place on Sunday, and photos and video from the event make it clear what they were celebrating.

Blue and pink balloons were tied down in arches ... as their celeb friends arrived. Dakota's folks -- Melanie Griffith and Don Johnson -- walked in, along with Sean Penn and Julia Roberts.

Looks like Don was last to arrive ... because about 10 minutes after he got there, the string of blue balloons were released into the sky. So, all signs point to them having a boy, unless someone screwed up the release.

Chris' big baby news came one week after Gwyneth Paltrow tied the knot with her new guy, Brad Falchuk. Chris didn't attend their wedding ... he was performing at the Global Citizen Festival in NYC, and Dakota was there with him.

Interesting, when Dakota walked onstage she was wearing a very baggy outfit and overcoat. Little did we know, she had a baby on board.