EXCLUSIVE

'Teen Mom' star Mackenzie McKee is expecting her third baby with husband Josh, but she'll be raising the kid as a single mom because they've finally called it quits.

McKee barely showed during her 18 week ultrasound. She's expecting a boy -- the due date is September 7th.

She says she's no longer living with Josh, the father of their 2-year-old daughter, Jaxie, and 4-year-old son, Gannon.

It's not the first time they've called it quits ... Josh walked out last April, but clearly came back.