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.
KK - KW - Surrogate - Woman - Birth
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....