Kim Kardashian and Kanye West have gone with a different surrogate to deliver their new baby -- but it ain't because they didn't like the last one ... she was simply unavailable.
Sources close to the couple tell TMZ ... Kim and Kanye hired a new woman to carry their fourth child, a boy ... not the 20-something-year-old they used last year for baby Chicago. The reason is simple ... that lady was already pregnant with a kid of her own.
We're told KK and KW's previous surrogate -- a young, healthy, fit woman in her 20s -- just gave birth to a daughter on Christmas Eve. Considering she'd just popped Chicago out less than 12 months prior, it's clear she and her husband of 5 years got busy quick.
BTW ... Kim and Kanye's first surrogate already has two boys of her own, so this is her third child.
No word on the profile of the Kardashian-Wests' new surrogate just yet, but ya gotta think she'll be comparable to the last one ... and Kim and Kanye will likely be just as involved during the pregnancy.
As we reported ... Kim and Kanye's new baby boy is due in early May. He'll be their fourth child in addition to North, Saint and Chi.